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/webglrenderingcontext.rs

@@ -222,7 +222,7 @@ impl WebGLRenderingContext {
         size: Size2D<u32>,
         attrs: GLContextAttributes,
     ) -> Result<WebGLRenderingContext, String> {
-        if pref!(webgl.testing.context_creation_error) {
+        if pref!(webgl_testing_context_creation_error) {
             return Err("WebGL context creation error forced by pref `webgl.testing.context_creation_error`".into());
         }