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/lexer.rs

@@ -1344,6 +1344,8 @@ pub enum LexicalErrorType {
     LineContinuationError,
     /// An unexpected end of file was encountered.
     Eof,
+    /// Occurs when a syntactically invalid assignment was encountered.
+    AssignmentError,
     /// An unexpected error occurred.
     OtherError(String),
 }
@@ -1389,6 +1391,7 @@ impl std::fmt::Display for LexicalErrorType {
                 write!(f, "unexpected character after line continuation character")
             }
             LexicalErrorType::Eof => write!(f, "unexpected EOF while parsing"),
+            LexicalErrorType::AssignmentError => write!(f, "invalid assignment target"),
             LexicalErrorType::OtherError(msg) => write!(f, "{msg}"),
         }
     }