Expose max_axis_index and max_axis_index for Vector2(i)

[aaronfranke]

EDIT 3: This also renames them to have _index at the end as per @reduz's suggestion.

Also some cleanup with core's Vector3 methods so that it is consistent, for example it returns enums internally (GDScript still gets ints).

EDIT: Also now fixes #34190.

EDIT 2: max_axis and min_axis were added to Vector2(i) internally with #48629, but this PR exposes them and uses the enum as the type.

[akien-mga]

Something is not working as the added enums don't appear in the docs and are not documented as return types.

[aaronfranke]

@akien-mga I'm not sure if that's possible to do with the current code for Variant types bound in variant_call.cpp. All other places in Godot that have enums as return types are classes that use ClassDB::bind_method.

[mhilbrunner]

Reviewed in PR meeting: fine to expose, but we should see about better naming for this both here and for Vector3i

Reduz suggestion:
get_min_axis_index and get_max_axis_index

[aaronfranke]

Done, but I used the names max_axis_index and max_axis_index instead of having get_ because none of the other Vector2/2i/3/3i methods start with get_. Ex: We have length not get_length.

[akien-mga]

Thanks!