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.