Implement type checker

[MarkusAmshove]

Check for TODO(type-check)

• #VAR MODIFIED #VAR has to be C
• #VAR SPECIFIED #VAR has to be OPTIONAL

[Claes65]

If you can implement this, then I can start type-checking in all statements. :)

If you look here: https://documentation.softwareag.com/natural/nat827mf/sm/synsym.htm#Operand_Definition_Table - you can see that other than types, we need to be able to determine if a variable is a literal, a group, an array, system variable, a reference to a DB field (statement label), a Dynamic definition or an arithmetic expression.. If a typed variable, we also need the Natural length of it (not the byte length). @MarkusAmshove