Allow non-numeric statement labels

When the allow non-numeric statement labels extension is enabled, Irie Pascal supports statement labels that look like identifiers (i.e. can contain letters and underscore characters).

For example, in the following program, loop is used as a statement label.

   program name(output);
   label loop;
   i : integer;
      i := 1;
      loop: writeln(i);
      i := i + 1;
      if i <= 20 then goto loop;