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/http_cache.rs

@@ -756,7 +756,7 @@ impl HttpCache {
     /// Storing Responses in Caches.
     /// <https://tools.ietf.org/html/rfc7234#section-3>
     pub fn store(&mut self, request: &Request, response: &Response) {
-        if pref!(network.http_cache.disabled) {
+        if pref!(network_http_cache_disabled) {
             return;
         }
         if request.method != Method::GET {