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/permissions.rs

@@ -301,7 +301,7 @@ pub(crate) fn get_descriptor_permission_state(
     // and let the user decide to grant the permission or not.
     let state = if allowed_in_nonsecure_contexts(&permission_name) {
         PermissionState::Prompt
-    } else if pref!(dom.permissions.testing.allowed_in_nonsecure_contexts) {
+    } else if pref!(dom_permissions_testing_allowed_in_nonsecure_contexts) {
         PermissionState::Granted
     } else {
         globalscope