Support extend on type bodypart

[Psithief]

Is your feature request related to a problem? Please describe.

To have custom body parts be treated as replacements for 'vanilla' body parts, we must edit the original body part, e.g. arm_l and add the custom part to the similar_bodyparts array.
Without extend, if multiple mods overwrite this field, then one will break.

Solution you would like.

Implement support for extend so mods don't have to clobber changes.

Describe alternatives you have considered.

No response

Additional context

See also #72331