Source view: add line number and per-line anchor

[RalfJung]

It would be useful for the source view to show line numbers that can also be used as link targets, similar to what the typical source view of rustdoc/GitLab/GitHub does.

I've been experimenting with the GitHub approach of using a table that holds the lines and the line numbers. Seems to work fairly well. Some issues with indentation.

I will finish this during my holidays next week and make a pr.

Current example

[jyn514]

@Liberatys did you finish your prototype? I'd be happy to see a PR even if it's not 100% done :)