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/webidls/XRSubImage.webidl

@@ -5,7 +5,7 @@
 // skip-unless CARGO_FEATURE_WEBXR
 
 // https://immersive-web.github.io/layers/#xrsubimagetype
-[SecureContext, Exposed=Window, Pref="dom.webxr.layers.enabled"]
+[SecureContext, Exposed=Window, Pref="dom_webxr_layers_enabled"]
 interface XRSubImage {
   [SameObject] readonly attribute XRViewport viewport;
 };