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/net/async_runtime.rs

@@ -19,9 +19,9 @@ pub static HANDLE: LazyLock<Mutex<Option<Runtime>>> = LazyLock::new(|| {
             .worker_threads(
                 thread::available_parallelism()
                     .map(|i| i.get())
-                    .unwrap_or(servo_config::pref!(threadpools.fallback_worker_num) as usize)
+                    .unwrap_or(servo_config::pref!(threadpools_fallback_worker_num) as usize)
                     .min(
-                        servo_config::pref!(threadpools.async_runtime_workers.max).max(1) as usize,
+                        servo_config::pref!(threadpools_async_runtime_workers_max).max(1) as usize,
                     ),
             )
             .enable_io()