Enable PDF files in preview pane (#9088)

## Summary of the Pull Request
This PR enables user to preview PDF files in the Explorer preview pane
and in Outlook. 

**What is this about:**
Windows does not support out of the box experience for previewing PDF
files in the preview pane. Users need to install third-party software
like Adobe Acrobat reader. The PdfPreviewHandler module enbales the user
to preview PDF files.

**How does someone test / validate:** 
Run the installer, open Explorer and select a PDF file, enable the
preview pane. Maybe need to remove third-party PDF software.

## Quality Checklist

- [X] **Linked issue:** #3548
- [ ] **Communication:** I've discussed this with core contributors in the issue. 
- [X] **Tests:** Added/updated and all pass
- [X] **Installer:** Added/updated and all pass
- [X] **Localization:** All end user facing strings can be localized
- [ ] **Docs:** Added/ updated
- [x] **Binaries:** Any new files are added to WXS / YML
   - [ ] No new binaries
   - [x] YML for signing
   - [x] WXS for installer

src/modules/previewpane/UnitTests-MarkdownPreviewHandler/MarkdownPreviewHandlerTest.cs

@@ -0,0 +1,116 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Drawing;
+using System.Windows.Forms;
+using Microsoft.PowerToys.PreviewHandler.Markdown;
+using Microsoft.PowerToys.STATestExtension;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using PreviewHandlerCommon;
+
+namespace MarkdownPreviewHandlerUnitTests
+{
+    [STATestClass]
+    public class MarkdownPreviewHandlerTest
+    {
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlAddsBrowserToFormWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithExternalImage.txt");
+
+                // Assert
+                Assert.AreEqual(2, markdownPreviewHandlerControl.Controls.Count);
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[0], typeof(WebBrowserExt));
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlAddsInfoBarToFormIfExternalImageLinkPresentWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithExternalImage.txt");
+
+                // Assert
+                Assert.AreEqual(2, markdownPreviewHandlerControl.Controls.Count);
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[1], typeof(RichTextBox));
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlAddsInfoBarToFormIfHTMLImageTagIsPresentWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithHTMLImageTag.txt");
+
+                // Assert
+                Assert.AreEqual(2, markdownPreviewHandlerControl.Controls.Count);
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[1], typeof(RichTextBox));
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlDoesNotAddInfoBarToFormIfExternalImageLinkNotPresentWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithScript.txt");
+
+                // Assert
+                Assert.AreEqual(1, markdownPreviewHandlerControl.Controls.Count);
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[0], typeof(WebBrowserExt));
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlUpdatesWebBrowserSettingsWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithExternalImage.txt");
+
+                // Assert
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[0], typeof(WebBrowserExt));
+                Assert.IsNotNull(((WebBrowser)markdownPreviewHandlerControl.Controls[0]).DocumentText);
+                Assert.AreEqual(DockStyle.Fill, ((WebBrowser)markdownPreviewHandlerControl.Controls[0]).Dock);
+                Assert.AreEqual(false, ((WebBrowser)markdownPreviewHandlerControl.Controls[0]).IsWebBrowserContextMenuEnabled);
+                Assert.AreEqual(true, ((WebBrowser)markdownPreviewHandlerControl.Controls[0]).ScriptErrorsSuppressed);
+                Assert.AreEqual(true, ((WebBrowser)markdownPreviewHandlerControl.Controls[0]).ScrollBarsEnabled);
+                Assert.AreEqual(false, ((WebBrowser)markdownPreviewHandlerControl.Controls[0]).AllowNavigation);
+            }
+        }
+
+        [TestMethod]
+        public void MarkdownPreviewHandlerControlUpdateInfobarSettingsWhenDoPreviewIsCalled()
+        {
+            // Arrange
+            using (var markdownPreviewHandlerControl = new MarkdownPreviewHandlerControl())
+            {
+                // Act
+                markdownPreviewHandlerControl.DoPreview<string>("HelperFiles/MarkdownWithExternalImage.txt");
+
+                // Assert
+                Assert.IsInstanceOfType(markdownPreviewHandlerControl.Controls[1], typeof(RichTextBox));
+                Assert.IsNotNull(((RichTextBox)markdownPreviewHandlerControl.Controls[1]).Text);
+                Assert.AreEqual(DockStyle.Top, ((RichTextBox)markdownPreviewHandlerControl.Controls[1]).Dock);
+                Assert.AreEqual(BorderStyle.None, ((RichTextBox)markdownPreviewHandlerControl.Controls[1]).BorderStyle);
+                Assert.AreEqual(Color.LightYellow, ((RichTextBox)markdownPreviewHandlerControl.Controls[1]).BackColor);
+                Assert.AreEqual(true, ((RichTextBox)markdownPreviewHandlerControl.Controls[1]).Multiline);
+            }
+        }
+    }
+}