[red-knot] Detect semantic syntax errors (#17463)

Summary
--

This PR extends semantic syntax error detection to red-knot. The main
changes here are:

1. Adding `SemanticSyntaxChecker` and `Vec<SemanticSyntaxError>` fields
to the `SemanticIndexBuilder`
2. Calling `SemanticSyntaxChecker::visit_stmt` and `visit_expr` in the
`SemanticIndexBuilder`'s `visit_stmt` and `visit_expr` methods
3. Implementing `SemanticSyntaxContext` for `SemanticIndexBuilder`
4. Adding new mdtests to test the context implementation and show
diagnostics

(3) is definitely the trickiest and required (I think) a minor addition
to the `SemanticIndexBuilder`. I tried to look around for existing code
performing the necessary checks, but I definitely could have missed
something or misused the existing code even when I found it.

There's still one TODO around `global` statement handling. I don't think
there's an existing way to look this up, but I'm happy to work on that
here or in a separate PR. This currently only affects detection of one
error (`LoadBeforeGlobalDeclaration` or
[PLE0118](https://docs.astral.sh/ruff/rules/load-before-global-declaration/)
in ruff), so it's not too big of a problem even if we leave the TODO.

Test Plan
--

New mdtests, as well as new errors for existing mdtests

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_parser/tests/fixtures.rs

@@ -504,10 +504,6 @@ impl<'a> SemanticSyntaxCheckerVisitor<'a> {
 }
 
 impl SemanticSyntaxContext for SemanticSyntaxCheckerVisitor<'_> {
-    fn seen_docstring_boundary(&self) -> bool {
-        false
-    }
-
     fn future_annotations_or_stub(&self) -> bool {
         false
     }