script: Preserve the order of entries when parsing an import map (#42754)

Unless the feature `preserve_order` is enabled, serde_json will sort the
map's entries in an alphanumerical order.
The specification instead expect the entries to be visited by the order
they are listed, a latter entry should replace a previous one when they
are resolved to the same url.

I've also removed a couple of `CanGc` introduced in #37405, unless I've
missed something there is not code that can trigger a GC.

Testing: A subtest now pass
Part of #37553

---------

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

components/script/dom/html/htmlscriptelement.rs

@@ -934,7 +934,6 @@ impl HTMLScriptElement {
                         ModuleOwner::Window(Trusted::new(self)),
                         Rc::clone(&text_rc),
                         base_url.clone(),
-                        can_gc,
                     );
                     let script = Script::ImportMap(ScriptOrigin::internal(
                         text_rc,