Add tracking of cabal files

[VeryMilkyJoe]

Allows tracking of cabal files so the cabal-fmt plugin on hls can access cabal files in order to format them.

[jneira]

I was just about to ask about this in the hls pr, many thanks!
Have you checked it works with the plugin?

[VeryMilkyJoe]

Yes, I built it via npm install, launched vscode and pressed f5 which launches a new vscode editor and then i opened a simple project in this editor and checked to see whether I could format a cabal file.
Cabal files are briefly underlined red upon opening them as they treated as haskell files by hls which I still need to fix.

[jneira]

nice, thank for the info, good to merge then

[fendor]

@jneira Please don't forget that this PR must not be published unless haskell/haskell-language-server#2047 has been merged, otherwise we will display errors for all users. We should probably write that down somewhere?

Maybe we also should think about a backwards compat story, as people might not immediately upgrade their HLS version to the latest available.

[jneira]

Hmm you are rigth, thanks for noting it. Could we do some change in the hls part (or the extension) to avoid those errors? To investigate.
Will do a release without this change in the meanwhile.

[fendor]

I think we can check in the vscode plugin for the version of the hls and hls-wrapper version to launch and conditionally enable loading of cabal files.