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_parser/src/lib.rs

@@ -123,6 +123,7 @@ use crate::lexer::LexResult;
 mod function;
 // Skip flattening lexer to distinguish from full ruff_python_parser
 mod context;
+mod invalid;
 pub mod lexer;
 mod parser;
 mod soft_keywords;