Changed all Plugin classes and PluginManager to be sealed, preventing further inheritance. This improves code safety and clarifies the intended usage of these classes.
#### 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.
Squashed commit of the following:
commit f7887aeda7
Author: Milly <milly.ca@gmail.com>
Date: Fri Jun 23 09:26:58 2017 +0900
Replace "&", "<", ">" to HTML entities in Markdown.
