Add a new node to the abstract symtax tree, which will let us do datatype checking of FB variable declarations using the standard algorithm, and no special cases.
Things that we must still check for in stage 3:
1) Handling of CONSTANTs:
1.a) "Any program organization unit attempts to modify the value of a variable that has been declared with the CONSTANT qualifier;"
1.b) From table 16.a "The CONSTANT qualifier shall not be used in the declaration of function block instances as described in 2.5.2.1."