ycmd identifier completion for Neovim.

THIS IS BARELY FUNCTIONING, LOOK AROUND, TRY IT IF YOU WANT BUT DON'T COMPLAINT IF IT DOESN'T DO WHAT YOU EXPECT!

• YouCompleteMe wrapped in a C++ msgpack-rpc binary.
• Neovim's tree-sitter API's based identifier extraction.
• As much as Lua as possible (but some VimL is still needed :/)
• Profit (at least for me).

Rationale

Why do I need this instead of using just YouCompleteMe?

Primarly because I wanted to experiment with Neovim specific technologies.

Right now YouCompleteMe does buffer completion like:

• send the buffer to the Python server
• extract the identifiers based on a regex for each language
• store the identifiers

I don't like that I need Python in order to use YouCompleteMe (plus I'm not that good at writing Python code even if I've done it for YouCompleteMe). This version uses the "binary" approach where I have a binary that I can pre-build for each platform where I work on, download it and be done with it. I wanted to try to use the tree-sitter APIs to extract the identifiers from a buffer.

This way each component will do what it does best where YouCompleteMe will simply returns the proper identifiers when it is asked for them.

What about LSP

I rely heavily on the buffer completion of YouCompleteMe at the moment while I don't use any LSP. If that time comes I will probably use the Neovim native capability anyway.

TODOs

• Actually use a saner CMake build for the C++. It will invole changing the CMake build of ycmd itself in order to expose a new target for the identifier only functionality.
• Put in place the whole CI/CD for the ycm binary (probably Github Actions).
• Find a way to properly test this thing. I would love to have something like the screen test of Neovim itself... maybe use exactly that!
• Integrate with native LSP client in order to provide YCM fuzzy lookup (maybe look at https://github.com/haorenW1025/completion-nvim).
• Try to move the VimL autocmd to the homemade lua API to experiment and prove the API (maybe if it does I could try to implement it directly in Neovim)
• Put in place statusline support for marking when we do not have ycm active due to missing parser or query
• Update to latest ycmd (they moved to C++17 completely and boost was removed entirely making part of my branch moot)
• Add the possibility to add blocklisted filetype (or lang) with a sane default list. Right now I complain everytime I open fzf.
• Do not activate when buffer is readonly