[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/ast.toml

@@ -104,7 +104,7 @@ fields = [{ name = "targets", type = "Expr*" }]
 doc = "See also [TypeAlias](https://docs.python.org/3/library/ast.html#ast.TypeAlias)"
 fields = [
   { name = "name", type = "Expr" },
-  { name = "type_params", type = "TypeParams?" },
+  { name = "type_params", type = "Box<crate::TypeParams>?" },
   { name = "value", type = "Expr" },
 ]