Reject more syntactically invalid Python programs (#8524)

## Summary

This commit adds some additional error checking to the parser such that
assignments that are invalid syntax are rejected. This covers the
obvious cases like `5 = 3` and some not so obvious cases like `x + y =
42`.

This does add an additional recursive call to the parser for the cases
handling assignments. I had initially been concerned about doing this,
but `set_context` is already doing recursion during assignments, so I
didn't feel as though this was changing any fundamental performance
characteristics of the parser. (Also, in practice, I would expect any
such recursion here to be quite shallow since the recursion is done on
the target of an assignment. Such things are rarely nested much in
practice.)

Fixes #6895

## Test Plan

I've added unit tests covering every case that is detected as invalid on
an `Expr`.

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/yield.py

@@ -22,8 +22,6 @@ def foo():
         pass
 
 
-    (yield a, b) = (1, 2)
-
     # some comment
     for e in l : yield e # some comment