Support generating LSIF and/or SCIP index.

[eownerdead]

Is your enhancement request related to a problem? Please describe.

Server Index Format (LSIF) and its successor SCIP Code Intelligence Protocol (SCIP) is a language-agnostic index format like ctags. It's developed by and used in sourcegraph, which provides web interfaces for code browsing with code navigation. Though there is already an LSIF implementation for Haskell, it has not maintained for long time.

Describe the solution you'd like

Adding a feature to generate SCIP index to this. Also, we can get LSIF index with the SCIP to LSIF converter

Describe alternatives you've considered

Implement as a separated project.

Additional context

I know that rust-analyzer had this functionality, but I can't find any document...

Discussion at clangd