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/window.rs

@@ -802,7 +802,7 @@ impl WindowMethods<crate::DomTypeHolder> for Window {
             // https://html.spec.whatwg.org/multipage/#script-closable
             let is_script_closable = (self.is_top_level() && history_length == 1) ||
                 is_auxiliary ||
-                pref!(dom.allow_scripts_to_close_windows);
+                pref!(dom_allow_scripts_to_close_windows);
 
             // TODO: rest of Step 3:
             // Is the incumbent settings object's responsible browsing context familiar with current?