Support `cabal.project` and `cabal.project.local` files

[fendor]

This extension currently provides syntax highlighting support for .cabal and .hs files and is recommended by the Haskell extension.

It would be great to extend this syntax highlighting support to cabal.project and cabal.project.local files, as HLS will start providing basic IDE features (such as diagnostics, etc...) relatively soon: https://github.com/haskell/haskell-language-server/pull/4615 The support itself should be relatively straight-forward, cabal.project files have the same structure and rules as .cabal files, so the only thing that truly needs to change are the keywords.

If you agree that we should have syntax highlighting for cabal.project, then the HLS team would be happy to contribute (and maintain if desired) the syntax for cabal.project files.

[fendor]

cc @rm41339 who kindly volunteered to implement the cabal.project syntax.