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/script_thread.rs

@@ -2794,7 +2794,7 @@ impl ScriptThread {
         self.background_hang_monitor.unregister();
 
         // If we're in multiprocess mode, shut-down the IPC router for this process.
-        if opts::multiprocess() {
+        if opts::get().multiprocess {
             debug!("Exiting IPC router thread in script thread.");
             ROUTER.shutdown();
         }