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/webgl2renderingcontext.rs

@@ -198,7 +198,7 @@ impl WebGL2RenderingContext {
 
     #[allow(unsafe_code)]
     pub(crate) fn is_webgl2_enabled(_cx: JSContext, global: HandleObject) -> bool {
-        if pref!(dom.webgl2.enabled) {
+        if pref!(dom_webgl2_enabled) {
             return true;
         }