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/testbinding.rs

@@ -880,17 +880,18 @@ impl TestBindingMethods<crate::DomTypeHolder> for TestBinding {
     fn PassVariadicAny(&self, _: SafeJSContext, _: Vec<HandleValue>) {}
     fn PassVariadicObject(&self, _: SafeJSContext, _: Vec<*mut JSObject>) {}
     fn BooleanMozPreference(&self, pref_name: DOMString) -> bool {
-        prefs::pref_map()
-            .get(pref_name.as_ref())
-            .as_bool()
+        prefs::get()
+            .get_value(pref_name.as_ref())
+            .try_into()
             .unwrap_or(false)
     }
     fn StringMozPreference(&self, pref_name: DOMString) -> DOMString {
-        prefs::pref_map()
-            .get(pref_name.as_ref())
-            .as_str()
-            .map(DOMString::from)
-            .unwrap_or_default()
+        DOMString::from_string(
+            prefs::get()
+                .get_value(pref_name.as_ref())
+                .try_into()
+                .unwrap_or_default(),
+        )
     }
     fn PrefControlledAttributeDisabled(&self) -> bool {
         false