[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/red_knot_python_semantic/src/semantic_index.rs

@@ -5,6 +5,7 @@ use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_index::{IndexSlice, IndexVec};
 
+use ruff_python_parser::semantic_errors::SemanticSyntaxError;
 use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
 use salsa::plumbing::AsId;
 use salsa::Update;
@@ -175,6 +176,9 @@ pub(crate) struct SemanticIndex<'db> {
 
     /// Map of all of the eager bindings that appear in this file.
     eager_bindings: FxHashMap<EagerBindingsKey, ScopedEagerBindingsId>,
+
+    /// List of all semantic syntax errors in this file.
+    semantic_syntax_errors: Vec<SemanticSyntaxError>,
 }
 
 impl<'db> SemanticIndex<'db> {
@@ -399,6 +403,10 @@ impl<'db> SemanticIndex<'db> {
             None => EagerBindingsResult::NotFound,
         }
     }
+
+    pub(crate) fn semantic_syntax_errors(&self) -> &[SemanticSyntaxError] {
+        &self.semantic_syntax_errors
+    }
 }
 
 pub struct AncestorsIter<'a> {