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/XRInputSource.webidl

@@ -19,7 +19,7 @@ enum XRTargetRayMode {
   "transient-pointer"
 };
 
-[SecureContext, Exposed=Window, Pref="dom.webxr.enabled"]
+[SecureContext, Exposed=Window, Pref="dom_webxr_enabled"]
 interface XRInputSource {
   readonly attribute XRHandedness handedness;
   readonly attribute XRTargetRayMode targetRayMode;
@@ -32,6 +32,6 @@ interface XRInputSource {
   [SameObject] readonly attribute Gamepad? gamepad;
 
   // Hand Input
-  [Pref="dom.webxr.hands.enabled"]
+  [Pref="dom_webxr_hands_enabled"]
   readonly attribute XRHand? hand;
 };