Support outputting `.fgd` entity definitions

[snake-biscuits]

Using .fgd for entity definitions could resolve some issues:

• Inheritance #4
• "model" in main tag conflicts with "model" key #9

NOTE: we already use ValveFGD in bloodhound.py

However, it'd involve a full rewrite of both bloodhound & fuse
Which they probably need, being loosely adapted from basic scripts
(need to break down into simpler functions)

afaik MRVN-Radiant .fgd support is a little spotty at the moment
So we'd need to figure that out and make a PR over there as well

If we could move towards a hybrid model (convert between formats, or produce both)
That'd be the a good solution for now

Outputting both formats lets us keep the current level of functionality in MRVN-Radiant
And .fgds could be used in other level editors

MRVN-Radiant's remap has a strict subset of the .map format it works with
but it will compile any .map that meets that spec (with some fiddling)