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/MediaDevices.webidl

@@ -5,7 +5,7 @@
 // https://w3c.github.io/mediacapture-main/#dom-mediadevices
 
 [Exposed=Window,
-SecureContext, Pref="dom.webrtc.enabled"]
+SecureContext, Pref="dom_webrtc_enabled"]
 interface MediaDevices : EventTarget {
     //                attribute EventHandler ondevicechange;
     Promise<sequence<MediaDeviceInfo>> enumerateDevices();
@@ -13,7 +13,7 @@ interface MediaDevices : EventTarget {
 
 partial interface Navigator {
     // [SameObject, SecureContext]
-    [Pref="dom.webrtc.enabled"] readonly        attribute MediaDevices mediaDevices;
+    [Pref="dom_webrtc_enabled"] readonly        attribute MediaDevices mediaDevices;
 };
 
 partial interface MediaDevices {