[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_parser/src/parser/statement.rs

@@ -974,7 +974,7 @@ impl<'src> Parser<'src> {
 
         ast::StmtTypeAlias {
             name: Box::new(name),
-            type_params,
+            type_params: type_params.map(Box::new),
             value: Box::new(value.expr),
             range: self.node_range(start),
         }