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.MarkdownViewer/Resources/highlight.js/styles/github-dark.min.css

@@ -0,0 +1,10 @@
+pre code.hljs{display:block;overflow-x:auto;padding:1em}code.hljs{padding:3px 5px}/*!
+  Theme: GitHub Dark
+  Description: Dark theme as seen on github.com
+  Author: github.com
+  Maintainer: @Hirse
+  Updated: 2021-05-15
+
+  Outdated base version: https://github.com/primer/github-syntax-dark
+  Current colors taken from GitHub's CSS
+*/.hljs{color:#c9d1d9;background:#0d1117}.hljs-doctag,.hljs-keyword,.hljs-meta .hljs-keyword,.hljs-template-tag,.hljs-template-variable,.hljs-type,.hljs-variable.language_{color:#ff7b72}.hljs-title,.hljs-title.class_,.hljs-title.class_.inherited__,.hljs-title.function_{color:#d2a8ff}.hljs-attr,.hljs-attribute,.hljs-literal,.hljs-meta,.hljs-number,.hljs-operator,.hljs-selector-attr,.hljs-selector-class,.hljs-selector-id,.hljs-variable{color:#79c0ff}.hljs-meta .hljs-string,.hljs-regexp,.hljs-string{color:#a5d6ff}.hljs-built_in,.hljs-symbol{color:#ffa657}.hljs-code,.hljs-comment,.hljs-formula{color:#8b949e}.hljs-name,.hljs-quote,.hljs-selector-pseudo,.hljs-selector-tag{color:#7ee787}.hljs-subst{color:#c9d1d9}.hljs-section{color:#1f6feb;font-weight:700}.hljs-bullet{color:#f2cc60}.hljs-emphasis{color:#c9d1d9;font-style:italic}.hljs-strong{color:#c9d1d9;font-weight:700}.hljs-addition{color:#aff5b4;background-color:#033a16}.hljs-deletion{color:#ffdcd7;background-color:#67060c}

QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer/Resources/highlight.js/styles/github.min.css

@@ -0,0 +1,10 @@
+pre code.hljs{display:block;overflow-x:auto;padding:1em}code.hljs{padding:3px 5px}/*!
+  Theme: GitHub
+  Description: Light theme as seen on github.com
+  Author: github.com
+  Maintainer: @Hirse
+  Updated: 2021-05-15
+
+  Outdated base version: https://github.com/primer/github-syntax-light
+  Current colors taken from GitHub's CSS
+*/.hljs{color:#24292e;background:#fff}.hljs-doctag,.hljs-keyword,.hljs-meta .hljs-keyword,.hljs-template-tag,.hljs-template-variable,.hljs-type,.hljs-variable.language_{color:#d73a49}.hljs-title,.hljs-title.class_,.hljs-title.class_.inherited__,.hljs-title.function_{color:#6f42c1}.hljs-attr,.hljs-attribute,.hljs-literal,.hljs-meta,.hljs-number,.hljs-operator,.hljs-selector-attr,.hljs-selector-class,.hljs-selector-id,.hljs-variable{color:#005cc5}.hljs-meta .hljs-string,.hljs-regexp,.hljs-string{color:#032f62}.hljs-built_in,.hljs-symbol{color:#e36209}.hljs-code,.hljs-comment,.hljs-formula{color:#6a737d}.hljs-name,.hljs-quote,.hljs-selector-pseudo,.hljs-selector-tag{color:#22863a}.hljs-subst{color:#24292e}.hljs-section{color:#005cc5;font-weight:700}.hljs-bullet{color:#735c0f}.hljs-emphasis{color:#24292e;font-style:italic}.hljs-strong{color:#24292e;font-weight:700}.hljs-addition{color:#22863a;background-color:#f0fff4}.hljs-deletion{color:#b31d28;background-color:#ffeef0}