Use hole fit plugins to record holes instead of parsing error messages

[wz1000]

Only works on 8.10+, so we would need to keep the parsing logic by CPPing the rest.

/cc @isovector I think this approach will provide a more robust way to get the local environment (TcGblEnv/TcLclEnv) at a hole, instead of going through LocalBindings. Thoughts? The tactic logic could work entirely inside TcRn.

[wz1000]

@pepeiborra do you think it is a good idea to use this approach on 8.10 and keep the parsing logic for earlier ghcs? Or should we just stick with a single code path throughout?

Hopefully we can get rid of all the parsing stuff for code actions when errors as values lands.

[pepeiborra]

Errors as values will take a while to arrive, so the plugin approach is good for 8.10, 9.0 and probably 9.2. By 9.4 we will be looking to migrate to errors as values, dropping support for 8.8 and hopefully avoiding having 3 code paths for dealing with hole fits.

On the other hand, if we think that errors as values is going to land in 9.2, then I would say don't bother.

[pepeiborra]

The ghcide Github project is becoming archived and merged into https://github.com/haskell/haskell-language-server

This PR will need to be reopened in the HLS repo. To do that, create a new branch from HLS HEAD in your HLS repo and do:

git remote add ghcide https://github.com/georgefst/ghcide.git
git fetch ghcide
git merge ghcide disable-warnings

Thanks!