Luau: support formatting user defined type functions

[Ukendio]

As noted by the title, the following error gets thrown error parsing: unexpected token function (START to END), expected type name. My guess is that the issue lies with the full moon dependency.

Edit: just saw this Kampfkarren/full-moon#334

I am thinking I could leave the issue open to be closed when you upgrade the version for fullmoon after the issue gets fixed there.