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/htmlscriptelement.rs

@@ -463,7 +463,7 @@ impl FetchResponseListener for ClassicContext {
         /*
         let options = unsafe { CompileOptionsWrapper::new(*cx, final_url.as_str(), 1) };
 
-        let can_compile_off_thread = pref!(dom.script.asynch) &&
+        let can_compile_off_thread = pref!(dom_script_asynch) &&
             unsafe { CanCompileOffThread(*cx, options.ptr as *const _, source_text.len()) };
 
         if can_compile_off_thread {