script: Merge Window::load_url with ScriptThread::navigate (#43668)

Both `Window::load_url` and `ScriptThread::navigate` implement parts of
the same navigate algorithm from the HTML spec
(https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigate).
`ScriptThread::navigate` had only one caller: `Window::load_url`.

This merges both methods into a single `navigate` function in
`navigation.rs`, which is the appropriate home for navigation logic. All
previous callers of `Window::load_url` now call `navigation::navigate`
directly.

Testing: This is a pure refactor with no behaviour changes, so no new
tests are needed. Existing tests cover the navigation paths affected.
Fixes: #43494

---------

Signed-off-by: thebabalola <t.babalolajoseph@gmail.com>

components/script/dom/html/htmlformelement.rs

@@ -86,6 +86,7 @@ use crate::dom::types::HTMLIFrameElement;
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::dom::window::Window;
 use crate::links::{LinkRelations, get_element_target, valid_navigable_target_name_or_keyword};
+use crate::navigation::navigate;
 use crate::script_runtime::CanGc;
 use crate::script_thread::ScriptThread;
 
@@ -1109,14 +1110,13 @@ impl HTMLFormElement {
             }
 
             // 4.2 Navigate targetNavigable to url
-            window
-                .root()
-                .load_url(
-                    NavigationHistoryBehavior::Push,
-                    false,
-                    load_data,
-                    CanGc::note(),
-                );
+            navigate(
+                &window.root(),
+                NavigationHistoryBehavior::Push,
+                false,
+                load_data,
+                CanGc::note(),
+            );
         });
 
         // 5. Set the form's planned navigation to the just-queued task.