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/net/filemanager_thread.rs

@@ -602,7 +602,7 @@ impl FileManagerStore {
         // Check if the select_files preference is enabled
         // to ensure process-level security against compromised script;
         // Then try applying opt_test_path directly for testing convenience
-        let opt_s = if pref!(dom.testing.html_input_element.select_files.enabled) {
+        let opt_s = if pref!(dom_testing_html_input_element_select_files_enabled) {
             opt_test_path
         } else {
             self.query_files_from_embedder(patterns, false, embedder_proxy)
@@ -632,7 +632,7 @@ impl FileManagerStore {
         // Check if the select_files preference is enabled
         // to ensure process-level security against compromised script;
         // Then try applying opt_test_paths directly for testing convenience
-        let opt_v = if pref!(dom.testing.html_input_element.select_files.enabled) {
+        let opt_v = if pref!(dom_testing_html_input_element_select_files_enabled) {
             opt_test_paths
         } else {
             self.query_files_from_embedder(patterns, true, embedder_proxy)