api: Flatten and simplify Servo preferences (#34966)

Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).

Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.

- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
  they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
  exposed by the Servo API.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/htmlmediaelement.rs

@@ -1325,7 +1325,7 @@ impl HTMLMediaElement {
             .render_poster_frame(image);
         self.upcast::<Node>().dirty(NodeDamage::OtherNodeDamage);
 
-        if pref!(media.testing.enabled) {
+        if pref!(media_testing_enabled) {
             self.owner_global()
                 .task_manager()
                 .media_element_task_source()