Improving Handling of Structured Diagnostics

Most of HLS is now using GHC structured diagnostics instead of regexes (#4605). However, there are still some messages we still rely on regex matching for, because they aren't available in structured format. I've created a [GHC issue](https://gitlab.haskell.org/ghc/ghc/-/issues/26353), but I'm not sure if this is a complete list.

## Known Instances

Here are the ones I know about:

**[Suggest Type Signature Change](https://github.com/haskell/haskell-language-server/blob/master/plugins/hls-change-type-signature-plugin/src/Ide/Plugin/ChangeTypeSignature.hs#L154)**: We have to extract the declName from a type mismatch from the `ErrInfo` (an `SDoc`).

**[Fill typed holes](https://github.com/haskell/haskell-language-server/pull/4695/files#diff-68eeecbcd90f79a3382f0c7c6bbdebd5e7495d52d821bc3c491742364009e125R108)**: The hole fit and refinement suggestions are stored in `ErrInfo` supplementary.

**[Fill type wildcard](https://github.com/haskell/haskell-language-server/blob/master/plugins/hls-refactor-plugin/src/Development/IDE/Plugin/Plugins/FillTypeWildcard.hs#L104)**: We need to determine whether the user wrote a top-level hole, for example whether the user wrote `foo :: _` or `foo :: Maybe _`. We do this again, by looking at `ErrInfo` context.

## Discussion

To move forward, we'll still need to

* Make sure the list above is complete
* Figure out specifically which fields we need to add

## Related Links

https://github.com/haskell/haskell-language-server/issues/4605
https://gitlab.haskell.org/ghc/ghc/-/issues/26353