Language server support for cabal.project files

It would be nice to have language server support for `cabal.project` files in the same manner as we provide for `.cabal` files.
My idea is to create a `hls-cabal-project-plugin` which should function similarly to the [`hls-cabal-plugin`](https://github.com/haskell/haskell-language-server/tree/master/plugins/hls-cabal-plugin).

Some features I have in mind:

- [ ] completions of
- [ ] keywords
- [ ] filepaths
- [ ] enum values
- [ ] diagnostics
- [ ] syntax highlighting