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/test.rs

@@ -89,15 +89,17 @@ fn test_argument_parsing_special() {
 // Helper function to test url
 fn test_url(input: &str, location: &str, cmdline_if_exists: &str, cmdline_otherwise: &str) {
     assert_eq!(
-        location_bar_input_to_url(input).unwrap().into_string(),
+        location_bar_input_to_url(input, "https://duckduckgo.com/html/?q=%s")
+            .unwrap()
+            .into_string(),
         location
     );
     assert_eq!(
-        get_default_url(Some(input), FAKE_CWD, |_| true).into_string(),
+        get_default_url(Some(input), FAKE_CWD, |_| true, &Default::default()).into_string(),
         cmdline_if_exists
     );
     assert_eq!(
-        get_default_url(Some(input), FAKE_CWD, |_| false).into_string(),
+        get_default_url(Some(input), FAKE_CWD, |_| false, &Default::default()).into_string(),
         cmdline_otherwise
     );
 }