Migrate change-type-signature-plugin to use structured diagnostics

[sgillespie]

This addresses using structured diagnostics from GHC (rather than regular expressions) for hls-change-type-signature-plugin.

A couple of notes:

• I reworked the regexp tests a little bit. Since I don't know how the original were created, I found it easier to generate them from the diagnostics of the existing golden test inputs. Happy to restore the old ones if that's better.
• I added some Traversals/Lenses in the plugin. Would it be more useful to put thim in Development.IDE.GHC.Compat.Error?

[fendor]

LGTM, thank you for taking the time and modernising the whole plugin!

Small comments, otherwise good to go, imo

[fendor]

We still need that? Should we open a GHC issue for adding the DeclName context to the error message?

[sgillespie]

We still need that?

Sadly, I think we do

Should we open a GHC issue for adding the DeclName context to the error message?

That would be awesome

[sgillespie]

@fendor do you want me to attempt to open a GHC issue for this?

[fendor]

LGTM!

[fendor]

Stack CI test failure is genuine, otherwise, let's merge!

[mergify]

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[sgillespie]

@Mergifyio requeue

[mergify]

requeue

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