Syntax Highlighting Nushell (.nu) files
### Proposal
It would be nice to have [Nushell](https://www.nushell.sh/) (.nu) files syntax highlighted.
### Links
- [Linguist grammar](https://github.com/hustcer/nu-grammar)
- [VSCode grammar](https://github.com/nushell/vscode-nushell-lang)
- [Treesitter grammar](https://github.com/nushell/tree-sitter-nu)
### Implementation plan
1. Add lexer to [Rouge](https://github.com/rouge-ruby/rouge/tree/master/lib/rouge/lexers).
- Uses regex to tokenize the input.
1. Add lexer to [Highlight.js](https://highlightjs.readthedocs.io/en/latest/language-contribution.html)
- Uses regex to tokenize the input.
1. Add lexer to [Monaco](https://microsoft.github.io/monaco-editor/monarch.html)
- Uses regex to tokenize the input.
1. Bump version of `rouge`. The version is defined in `Gemfile.lock`.
1. Bump version of `highlight.js`. The version is defined in `yarn.lock`.
1. Bump version of `monaco`. The version is defined in `yarn.lock`.
### Verification
Verify that syntax highlighting works correctly in:
- repository view and diffs
- snippets
- markdown rendering
issue