Adhoc handling of custom entries for notations and improvements on plugins (VernacExtend)

[pcarrott]

• To handle InCustomEntry from the AST, only consider notations with a non-empty pattern for the step context.
• A single notation might yield multiple Locate results. We now iterate over all, instead of just trying the first one.
• AST nodes with GenArg now (partially) support ListArg, besides ExtraArg.
• Plugin for Coq-Equations is now recognized by FileContext.

[Nfsaavedra]

Are there any simple examples to test what was changed? At least the second and third point. I believe the last one would require adding Coq-Equations to our test setup.

[Nfsaavedra]

LGTM! However, we should probably wait for the issue with the menhir package to be fixed, re-run the CI, and only then merge.