[syntax-errors] Duplicate type parameter names (#16858)

Summary
--

Detects duplicate type parameter names in function definitions, class
definitions, and type alias statements.

I also boxed the `type_params` field on `StmtTypeAlias` to make it
easier to
`match` with functions and classes. (That's the reason for the red-knot
code
owner review requests, sorry!)

Test Plan
--

New `ruff_python_syntax_errors` unit tests.

Fixes #11119.

crates/ruff_python_ast/src/generated.rs

@@ -6494,7 +6494,7 @@ pub struct StmtDelete {
 pub struct StmtTypeAlias {
     pub range: ruff_text_size::TextRange,
     pub name: Box<Expr>,
-    pub type_params: Option<crate::TypeParams>,
+    pub type_params: Option<Box<crate::TypeParams>>,
     pub value: Box<Expr>,
 }