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/local_directory_listing.rs

@@ -16,7 +16,7 @@ use servo_url::ServoUrl;
 use url::Url;
 
 pub fn fetch(request: &mut Request, url: ServoUrl, path_buf: PathBuf) -> Response {
-    if !pref!(network.local_directory_listing.enabled) {
+    if !pref!(network_local_directory_listing_enabled) {
         // If you want to be able to browse local directories, configure Servo prefs so that
         // "network.local_directory_listing.enabled" is set to true.
         return Response::network_error(NetworkError::Internal(