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/UnitTests-MarkdownPreviewHandler.csproj

@@ -0,0 +1,69 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Platforms>x64</Platforms>
+    <AssemblyTitle>UnitTests-MarkdownPreviewHandler</AssemblyTitle>
+    <AssemblyDescription>PowerToys UnitTests-MarkdownPreviewHandler</AssemblyDescription>
+    <AssemblyCompany>Microsoft Corp.</AssemblyCompany>
+    <AssemblyCopyright>Copyright (C) 2020 Microsoft Corp.</AssemblyCopyright>
+    <AssemblyProduct>PowerToys</AssemblyProduct>
+    <AssemblyTitle>UnitTests-MarkdownPreviewHandler</AssemblyTitle>
+    <Company>Microsoft Corp.</Company>
+    <Product>PowerToys</Product>
+    <NeutralLanguage>en-US</NeutralLanguage>
+    <Description>PowerToys UnitTests-MarkdownPreviewHandler</Description>
+    <Copyright>Copyright (C) 2020 Microsoft Corp.</Copyright>
+  </PropertyGroup>
+  
+  <PropertyGroup>
+    <ProjectGuid>{A2B51B8B-8F90-424E-BC97-F9AB7D76CA1A}</ProjectGuid>
+    <RootNamespace>PreviewPaneUnitTests</RootNamespace>
+    <AssemblyName>PreviewPaneUnitTests</AssemblyName>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <ProjectTypeGuids>{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
+    <ReferencePath>$(ProgramFiles)\Common Files\microsoft shared\VSTT\$(VisualStudioVersion)\UITestExtensionPackages</ReferencePath>
+    <IsCodedUITest>False</IsCodedUITest>
+  </PropertyGroup>
+
+  <Import Project="..\..\..\Version.props" />
+  
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.10.0" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Markdig.Signed" Version="0.22.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="3.3.0">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="MSTest.TestAdapter" Version="2.2.5" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.2.5" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\common\PreviewHandlerCommon.csproj" />
+    <ProjectReference Include="..\MarkdownPreviewHandler\MarkdownPreviewHandler.csproj" />
+  </ItemGroup>
+  <ItemGroup>
+    <Content Include="HelperFiles\MarkdownWithExternalImage.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+    <Content Include="HelperFiles\MarkdownWithHTMLImageTag.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+    <Content Include="HelperFiles\MarkdownWithscript.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="..\..\..\codeAnalysis\GlobalSuppressions.cs" Link="GlobalSuppressions.cs" />
+    <Compile Include="..\STATestClassAttribute.cs" Link="STATestClassAttribute.cs" />
+    <Compile Include="..\STATestMethodAttribute.cs" Link="STATestMethodAttribute.cs" />
+    <AdditionalFiles Include="..\..\..\codeAnalysis\StyleCop.json">
+      <Link>StyleCop.json</Link>
+    </AdditionalFiles>
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="StyleCop.Analyzers" Version="1.1.118">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+</Project>