Add plugin to format .cabal files

[jneira]

• From Format cabal files vscode-haskell#241
• I could be implemented using cabal-fmt as suggested by @fendor

/cc @WillSewell

[WillSewell]

Or use cabal format, which I see as the "official" style.

[fendor]

cabal-fmt does quite a bit more than cabal format, but I guess you are right that it is more official (although cabal-fmt is produced by a cabal maintainer).
Also, in my opinion the bigger issue, is that with tools such as stack, rules_haskell, and similar, we might not have cabal installed on the system. Maybe we can extend hls to have multiple cabal formatters as well?

[maoe]

Sadly cabal format expands common stanzas (haskell/cabal#5734) so I don't think it's suitable as a general auto-formatter.

[Avi-D-coder]

Off topic: Thanks for mentioning this!

Sadly cabal format expands common stanzas (haskell/cabal#5734) so I don't think it's suitable as a general auto-formatter.

I believe this is will allow https://github.com/Avi-D-coder/implicit-hie to support common stanza!

[jneira]

I wonder if handling of cabal config should live in its own lsp server, to not bloat hls with cabal related dependencies and features

[ndmitchell]

I imagine that eventually we'll want to do things like creating new modules and automatically adding them to Cabal, or marking whether a module is exposed or not at the top of the module and adjusting that quickly. That requires a sufficient level of integration with Cabal that I suspect eventually we'll have to become a Cabal LSP (and potentially a Stack LSP). That might be a decade away though :)