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