Stricter checks for multiple assignment syntax

[HertzDevil]

Fixes #11442. Fixes #12911.

Also disallows the following:

# block in setter method target without receiver, ignored by parser
a {}, b = ...

# block in setter method target with receiver, ignored by literal expander
a.b {}, c = ...

# empty parentheses in setter method target with receiver, ignored by parser
a.b(), c = ...