gh-156058: Disallow lone starred expressions at the grammar level - #156059

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 were allowed by `python.gram` and only rejected
in the codegen step. This means that the
documented "Full Grammar specification" was incomplete.

Change the grammar itself to disallow "lone" starred
expressions.

This interfered with the `invalid_legacy_expression`
rule, where in ``return i*i for _ in _`` the ``i *i``
was parsed similarly to ``print *i``, generating
"can't use starred expression here" from `*i` (before
getting to the "Missing parentheses in call" message
instead of a generic "invalid syntax".
Exclude the star in `invalid_legacy_expression` to
prevent this.

Co-authored-by: Blaise Pabon <blaise@gmail.com>

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