Consistent naming of substack inputs

[tmickel]

In our control_repeat block, we name the substack input "DO"; in control_forever, it's named "NAME".

I suggest we rename the first substack in all blocks "SUBSTACK" and the second substack for if-else "SUBSTACK2" for consistency with the usual Scratch interpreter/sb2.

[tmickel]

@thisandagain Did you have thoughts on this?

[thisandagain]

I like using SUBSTACK consistently across repeat and forever blocks. I'm not a huge fan of SUBSTACK2 for if/else but do like the consistency across grammars. 👍