Handle type aliases in empty file

[thomasmarshall]

For example:

#: type foo = Integer

This is currently ignored, does not generate a type alias in either the whitequark or Prism RBS rewriters.

[amomchilov]

This should be pretty simple in the Prism case, just a matter of creating a statements node for the top PM_PROGRAM_NODE node

[KaanOzkan]

Whitequark will be tougher, it seems that it can't even parse the file and see the comment. We'll have to call the emit_comment earlier

[jesse-shopify]

Stripe has told us they'll be working on the removal of legacy parser from Prism desugaring this week. I think it might be better to tackle that after there is less distance between the queue and main.