Disallow lone starred expressions at the *grammar* level

Statements like the following (where () can be any iterable -- or at the syntax level, any value):

• *()
• x = *()
• yield *()
• return *()
• for _ in *(): ...

are not valid without a comma after the iterable.
These are allowed by python.gram and only rejected in the code generation step. This means that the documented Full Grammar specification is incomplete. We can do better.

Linked PRs

• gh-156059