[Peek][PreviewPane]Show Copy entry in right-click copy menu (#33845)

## Summary of the Pull Request
Fixes two bugs:
- Peek: Missing "Copy" menu-item for all WebView2 previewers.
- PreviewPane: Missing "Copy" menu-item for markdown files only.

## Detailed Description of the Pull Request / Additional comments
The issues are:
- Peek: 
- When not using Monaco (markdown, html) - the default WebView2 context
menu has been disabled. I have enabled it and then disabled ALL
menu-items other than "Copy" (such as "Back").
- When using Monaco + Release (other code files) - current code tries to
use the Monaco context menu, but it is somehow disabled at runtime. I
spent MANY hours trying to find out why but without success. It works
fine when I view the generated html + js files in a browser or in a
Debug build or in PreviewPane. But I couldn't find the root cause.
Trying to fix it by enabling the WebView2 context menu instead doesn't
work as for whatever reason, WebView2 doesn't generate a "Copy"
menu-item (it thinks there's no selected text when there is). So in this
case, the only thing I could get to work was generating context
menu-items via WebView2 callbacks that call JS functions. As a bonus,
this way of doing it also allows "Toggle text wrapping" to work.
- PreviewPane:
- Markdown - the default WebView2 context menu has been disabled. Like
for Peek, I have enabled it and then disabled ALL menu-items other than
"Copy" (such as "Back").
- Monaco (other code files) - this already just works fine, so I've left
it as is. I *could* make it work the same way as I've done for Peek for
consistency, but I've chosen to leave it as is since it works.
  

![image](https://github.com/user-attachments/assets/d758ada7-bb62-4f40-bef7-ad08ffb83786)

![image](https://github.com/user-attachments/assets/4e0baa7e-632f-412a-b2b1-b9f666277ca7)

src/modules/peek/Peek.FilePreviewer/Previewers/WebBrowserPreviewer/WebBrowserPreviewer.cs

@@ -43,6 +43,9 @@ namespace Peek.FilePreviewer.Previewers
         [ObservableProperty]
         private bool isDevFilePreview;
 
+        [ObservableProperty]
+        private bool customContextMenu;
+
         private bool disposed;
 
         public WebBrowserPreviewer(IFileSystemItem file, IPreviewSettings previewSettings)
@@ -107,9 +110,14 @@ namespace Peek.FilePreviewer.Previewers
                 {
                     bool isHtml = File.Extension == ".html" || File.Extension == ".htm";
                     bool isMarkdown = File.Extension == ".md";
-                    IsDevFilePreview = MonacoHelper.SupportedMonacoFileTypes.Contains(File.Extension);
 
-                    if (IsDevFilePreview && !isHtml && !isMarkdown)
+                    bool supportedByMonaco = MonacoHelper.SupportedMonacoFileTypes.Contains(File.Extension);
+                    bool useMonaco = supportedByMonaco && !isHtml && !isMarkdown;
+
+                    IsDevFilePreview = supportedByMonaco;
+                    CustomContextMenu = useMonaco;
+
+                    if (useMonaco)
                     {
                         var raw = await ReadHelper.Read(File.Path.ToString());
                         Preview = new Uri(MonacoHelper.PreviewTempFile(raw, File.Extension, TempFolderPath.Path, _previewSettings.SourceCodeTryFormat, _previewSettings.SourceCodeWrapText, _previewSettings.SourceCodeStickyScroll, _previewSettings.SourceCodeFontSize));