Improved MarkdownViewer

#### Features
- Uses the latest [github-markdown.css](https://github.com/sindresorhus/github-markdown-css/blob/main/github-markdown.css) file that contains styling support for both dark and light mode.
- Table of Contents has an improved design, and:
	- The width can be resized.
	- TOC headings are automatically highlighted to help track your position in the file.
- Improved fullscreen layout based on Github.com behavior.
- External links clicked will open in default browser instead of doing nothing.
- Uses [markdownItAnchor](https://github.com/valeriangalliat/markdown-it-anchor) to allow heading anchor links in the file to work.
- Uses [highlight.js](https://github.com/highlightjs/highlight.js) to provide syntax highlighting to codeblocks.
#### Changes
- Made changes to allow the `md2html.html` file to use relative file imports for better maintainability.
- MarkdownViewer can now easily be customized by users by modifying files in `<Quicklook data folder>/QuickLook.Plugin.MarkdownViewer/`
- Caching and `localStorage` is now supported thanks to these changes.
- Prevent default behavior of spacebar scrolling the page, while we use spacebar to dismiss the preview.
- Sets `WebView` `DefaultBackgroundColor` to prevent white flash in dark mode. After the page has loaded, sets `DefaultBackgroundColor` back to white to have the expected default behavior on HTML pages that don't specify any background color.
#### Clean up
- Removed the need for `jQuery`.
- Removed old polyfill code.

QuickLook.Plugin/QuickLook.Plugin.HtmlViewer/WebpagePanel.cs

@@ -1,4 +1,4 @@
-// Copyright © 2021 Paddy Xu and Frank Becker
+// Copyright © 2021 Paddy Xu and Frank Becker
 //
 // This file is part of QuickLook program.
 //
@@ -17,6 +17,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Drawing;
 using System.IO;
 using System.Reflection;
 using System.Windows;
@@ -29,8 +30,8 @@ namespace QuickLook.Plugin.HtmlViewer
 {
     public class WebpagePanel : UserControl
     {
-        private Uri _currentUri;
-        private WebView2 _webView;
+        public Uri _currentUri;
+        public WebView2 _webView;
 
         public WebpagePanel()
         {
@@ -44,10 +45,12 @@ namespace QuickLook.Plugin.HtmlViewer
                 {
                     CreationProperties = new CoreWebView2CreationProperties
                     {
-                        UserDataFolder = Path.Combine(SettingHelper.LocalDataPath, @"WebView2_Data\\")
-                    }
+                        UserDataFolder = Path.Combine(SettingHelper.LocalDataPath, @"WebView2_Data\\"),
+                    },
+                    DefaultBackgroundColor = OSThemeHelper.AppsUseDarkTheme() ? Color.FromArgb(255, 32, 32, 32) : Color.White, // Prevent white flash in dark mode
                 };
                 _webView.NavigationStarting += NavigationStarting_CancelNavigation;
+                _webView.NavigationCompleted += WebView_NavigationCompleted;
                 Content = _webView;
             }
         }
@@ -83,6 +86,11 @@ namespace QuickLook.Plugin.HtmlViewer
             if (newUri != _currentUri) e.Cancel = true;
         }
 
+        private void WebView_NavigationCompleted(object sender, CoreWebView2NavigationCompletedEventArgs e)
+        {
+            _webView.DefaultBackgroundColor = Color.White; // Reset to white after page load to match expected default behavior
+        }
+
         public void Dispose()
         {
             _webView?.Dispose();