func_instance support

[Fennecai]

Is your feature request related to a problem? Please describe.
sort-of. the addon doesn't import func_instance- meaning a lot of stuff in vmf imports goes missing. this goes for both the old one and the new "plumber"

Is your feature request related to a specific game? Please specify.
nope.

Describe the solution you'd like
just import the contents of the func_instance entities by loading in the VMfs they point to; with the transform of the entity used as an offset.

Additional context
a lot of instances in my maps are used for lighting, doors, etc. so it'd be great to have them so the scene looks right in blender without too much additional tweaking