Make getAllTypes unwrap IDL record<K, V> types (#37039)

IDL `record` types can themselves contain types that are not described
anywhere else. An example is in
https://github.com/servo/servo/issues/37038, where the `record` contains
a definition of a union. These inner types must be returned from
`getAllTypes`, otherwise we won't generate code for them.

This PR also adds a few type annotations. I can remove them if
requested, but I think they're helpful.

Testing:  Includes a regression test
Fixes: https://github.com/servo/servo/issues/37038

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/script_bindings/webidls/TestBinding.webidl

@@ -621,3 +621,9 @@ namespace TestNS {
 };
 
 typedef Promise<undefined> PromiseUndefined;
+
+// https://github.com/servo/servo/issues/37038
+dictionary NotUsedAnyWhereElse {};
+dictionary RecordFieldWithUnionInside {
+    record<USVString, (USVString or NotUsedAnyWhereElse)> recordWithUnionField;
+};