Add Indexable#index! overloads with offset parameter

[HertzDevil]

Follow-up to #11566. Indexable#index overrides Enumerable#index and has an extra parameter, so it makes sense the raising variant should be overridden in a similar way. (There are further overrides in Indexable's including types, but they do not introduce any new signatures, so this PR will suffice.)

[straight-shoota]

There is a discrepancy between the parameter name in code (object) and in the docs (value). #index uses object, so that seems to be the natural choice.

[HertzDevil]

The base overload actually uses obj. Maybe we should unify those parameter names here too?

[straight-shoota]

I'd prefer object over obj and I'd rather standardize on that.

Thumbs up please, if you agree =)

[HertzDevil]

No other methods in Enumerable or Indexable use object as a parameter name, but it appears in many container methods, e.g. Array#insert and Set#<< (some similar methods use value instead, which is what the docs here originally put).

On the other hand, obj also shows up in Enumerable#includes? and #each_with_object, plus similar ones in Iterator and Iterable.

[HertzDevil]

I don't think it would be wise to do any renamings on the whole standard library in here, so to move forward I suggest using obj here, since that's what the parameter is called in Enumerable#index!, and revert everything else.