Support generating SCIP/LSIF index

Nobody has claimed this yet.

The [Language Server Index Format](https://lsif.dev) and its (kind of) successor [SCIP](https://github.com/sourcegraph/scip) define language-independent formats for information dumps of a language server. It allows tools like code editors and online repositories (eg. supported by GitHub, GitLab) to provide some LSP features without requiring an entire language server instance running in the background.

The [LSIF specification](https://microsoft.github.io/language-server-protocol/specifications/lsif/0.6.0/specification/) has currently not reached 1.0.0 status yet.
In its most recent specification, LSIF would allow for the following language features in offline situations:

- `textDocument/definition`
- `textDocument/declaration`
- `textDocument/hover`
- `textDocument/references`
- `textDocument/implementation`
- `textDocument/typeDefinition`
- `textDocument/foldingRange`
- `textDocument/documentLink`
- `textDocument/documentSymbol`
- `textDocument/semanticTokens/full`

I'm not entirely sure whether this topic has been discussed in this community before, I wasn't able to find any mentions here in the older issues. In my opinion it could be a great feature to have a way of extracting project information out of the LuaLS into LSIF files.

No source file, test, or entry point is named. Start by comparing the LSIF and SCIP specifications with the information LuaLS already exposes, then determine the scope of generating project indexes. Done means LuaLS can extract project information into a supported index format and the behavior is covered by tests.

Written by the indexing model from the issue text.