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>

tests/wpt/webgl/meta-legacy-layout/conformance/__dir__.ini

@@ -1 +1 @@
-prefs: ["dom.offscreen_canvas.enabled:true","dom.imagebitmap.enabled:true"]
+prefs: ["dom_offscreen_canvas_enabled:true","dom_imagebitmap_enabled:true"]

tests/wpt/webgl/meta-legacy-layout/conformance2/__dir__.ini

@@ -1,2 +1,2 @@
-prefs: ["dom.webgl2.enabled:true","dom.offscreen_canvas.enabled:true","dom.imagebitmap.enabled:true"]
+prefs: ["dom_webgl2_enabled:true","dom_offscreen_canvas_enabled:true","dom_imagebitmap_enabled:true"]
 

tests/wpt/webgl/meta-legacy-layout/conformance2/offscreencanvas/__dir__.ini

@@ -1 +1 @@
-prefs: ["dom.offscreen_canvas.enabled:true"]
+prefs: ["dom_offscreen_canvas_enabled:true"]

tests/wpt/webgl/meta/__dir__.ini

@@ -1 +1 @@
-prefs: ["dom.offscreen_canvas.enabled:true", "dom.imagebitmap.enabled:true", "dom.webgl2.enabled:true"]
+prefs: ["dom_offscreen_canvas_enabled:true", "dom_imagebitmap_enabled:true", "dom_webgl2_enabled:true"]

tests/wpt/webgl/meta/conformance2/offscreencanvas/__dir__.ini

@@ -1 +1 @@
-prefs: ["dom.offscreen_canvas.enabled:true"]
+prefs: ["dom_offscreen_canvas_enabled:true"]