Add lean4 support

[ghost]

Lean4
Lean is a functional programming language that makes it easy to write correct and maintainable code. You can also use Lean as an interactive theorem prover.

Additional resources

• https://lean-lang.org/
• Lean Manual https://lean-lang.org/lean4/doc/

[BachoSeven]

I second this, as it's a fast-growing language in its area.

[reynie1514]

Please add this!

[rzeta0]

Is there any progress on lean support in prism?

The 3rd party lean definition for highlight.js doesn't work when used from a CDN and the bug has been left open for over a year.

[ydewit]

Sorry, I did not realize this issue had already been created. I created a new one here: #3809 I will close it.

The only thing I would add here is the actual tmLanguage file. If that helps in any way.