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>

ports/servoshell/egl/android/resources.rs

@@ -16,7 +16,6 @@ impl ResourceReaderInstance {
 impl ResourceReaderMethods for ResourceReaderInstance {
     fn read(&self, res: Resource) -> Vec<u8> {
         Vec::from(match res {
-            Resource::Preferences => &include_bytes!("../../../../resources/prefs.json")[..],
             Resource::HstsPreloadList => {
                 &include_bytes!("../../../../resources/hsts_preload.json")[..]
             },