script: Focus on mousedown instead of mouse click according to spec (#38589)

- Focus on mousedown instead of mouse click according to
[spec](https://w3c.github.io/uievents/#handle-native-mouse-down)
- Refactor to follow spec closer and make things more clear.
- Add some spec link.
- Remove some dead spec link.

Still some preparation before implementing #38435.
Testing: No regression in WebDriver & WPT. But update some outdated
test.
Fixes: #38588

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/script/dom/mouseevent.rs

@@ -222,6 +222,7 @@ impl MouseEvent {
     }
 
     /// Create a [MouseEvent] triggered by the embedder
+    /// <https://w3c.github.io/uievents/#create-a-cancelable-mouseevent-id>
     pub(crate) fn for_platform_mouse_event(
         event: embedder_traits::MouseButtonEvent,
         pressed_mouse_buttons: u16,