Allow macro_identifier everywhere an identifier is

[d-alonso]

As you might be aware, I can't test this in this repo because it involves setting an env var to communicate with the external scanner.

This substitutes everywhere an $.identifier was with a new hidden rule that admits a $.macro_identifier as well. This fixes a parsing ambiguity when parsing this code:

subroutine parse_dict()
  allocate(GridEntity :: new_entity)
end subroutine parse_dict

When setting GridEntity as a macro identifier, the parser would first assume that it is an allocate statement without the optional type part, and that GridEntity is the name of the allocation place, and then end in ERROR due to the unexpected :: part after.

It also hopefully ends other possible ambiguities that were not discovered yet