REF: collect methods in NumericIndex
Seperates the refactoring of existing methods out of #41153. Will make it easier to see what #41153 actually brings in of new things.

This PR changes no functionality, but only makes the exisiting numeric indexes into thin subclasses of the existing `NumericIndex`.
Updated. LGTM; if you find anywhere where `@final` can be correctly added, pls do Wrt. @final, I’m not sure I like it in library code.

Couldn’t a third party subclass an index and change one of methods in an unanticipated way, but the @final be in the way of that? Updated type hints. thanks @topper-123 