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/webgpu/wgpu_thread.rs

@@ -127,7 +127,7 @@ impl WGPU {
         external_images: Arc<Mutex<WebrenderExternalImageRegistry>>,
         wgpu_image_map: WGPUImageMap,
     ) -> Self {
-        let backend_pref = pref!(dom.webgpu.wgpu_backend);
+        let backend_pref = pref!(dom_webgpu_wgpu_backend);
         let backends = if backend_pref.is_empty() {
             wgt::Backends::PRIMARY
         } else {