Support syntax highlighting in Markdown fenced code blocks

[TUSF]

Currently, if you have a Markdown file with:

```zig
// some code
```

VSCode won't know how to highlight the block even with the extension, neither in-editor, nor in preview.

Taking a look over at this page:

To implement such support for a language, typically, two kinds of TextMate language grammars are needed: One defines language grammars for the language; the other provides scope redirection. Following TextMate's naming conventions is important for them.

As for the preview, that uses highlight.js, and would probably require a plugin of such to be inserted into the rendered HTML? Dunno if that's possible.

[Vexu]

This was previously implemented in #66 but was reverted for breaking syntax highlighting in general #69