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>

ports/servoshell/panic_hook.rs

@@ -41,6 +41,8 @@ pub(crate) fn panic_hook(info: &PanicHookInfo) {
     }
     drop(stderr);
 
+    // TODO: This shouldn't be using internal Servo options here. Perhaps this functionality should
+    // move into libservo itself.
     if opts::get().hard_fail && !opts::get().multiprocess {
         // When we are exiting due to a hard-failure mode, we trigger a segfault so that crash
         // tests detect that we crashed. If we exit normally it just looks like a non-crash exit.