Feat: syntax highlighting in markdown code blocks

[LoicRiegel]

Feature: add syntax highlighting inside nushell codeblocks in markdown files

Issue #192

Adds syntax highlighting for nushell code blocks when editing markdown files inside VS code

The code is very inspired from this repo.

⚠️ The syntax highlighting still is not available when opening the Preview of the md files in VS code. VS code uses something else (highlight.js) so that will be done and somewhere else I guess.

How to test

1. Clone the branch
2. Start debugging (F5) in VS Code
3. Open a md files, create a nushell code block and start typing nushell code

Result

[fdncred]

I don't understand. What's the point of this PR if it doesn't highlight the text in markdown?

[LoicRiegel]

It does while editing markdown files (left part of the screenshot). Currently we don't have syntax highlighting injected here, only while editing .nu files.

What still does not work is the right of the screenshot, the preview mode.