Extension to elaborator reflection interface to enable automated derivation of decidable equality

[bhaktishh]

Description

This PR adds a function to the elaborator reflection interface, to enable automated derivation of decidable equality (in library idris2-elab-util).

The function getDecEqConPairs expects a type, and returns a pair (List Nat, List (String, String)), which represent the indices of the type arguments that are parameters, and pairs of constructor names (as strings) that can have the same type.

For example, for the type Vect, the function would return ([1], [("Nil", "Nil"), ("::", "::")]).

Self-check

• This is my first time contributing, I've carefully read CONTRIBUTING.md
  and I've updated CONTRIBUTORS with my name.
• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md
• I confirm that this contribution did not involve GenerativeAI nor Large Language Models.

[gallais]

As I said, my main issue with this PR is that it is very ad-hoc for this specific use-case.
However, Bhakti did claim it would help derive equality for not just ADTs but families
so given the proper test cases (see e.g. DeriveFunctor.idr for an extensive test suite
documenting both where it works and where it fails), I'd be OK merging it in.

I still think there is a better way (via a view isolating the diagonal) but if this lands
features today it's better than an ideal solution in X years.

Edit: FTR and to my own surprise, I was not actually unsub'd from this PR.