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/tests/filemanager_thread.rs

@@ -15,16 +15,19 @@ use net_traits::blob_url_store::BlobURLStoreError;
 use net_traits::filemanager_thread::{
     FileManagerThreadError, FileManagerThreadMsg, ReadFileProgress,
 };
-use servo_config::set_pref;
+use servo_config::prefs::Preferences;
 
 use crate::create_embedder_proxy;
 
 #[test]
 fn test_filemanager() {
+    let mut preferences = Preferences::default();
+    preferences.dom_testing_html_input_element_select_files_enabled = true;
+    servo_config::prefs::set(preferences);
+
     let pool = CoreResourceThreadPool::new(1, "CoreResourceTestPool".to_string());
     let pool_handle = Arc::new(pool);
     let filemanager = FileManager::new(create_embedder_proxy(), Arc::downgrade(&pool_handle));
-    set_pref!(dom.testing.html_input_element.select_files.enabled, true);
 
     // Try to open a dummy file "components/net/tests/test.jpeg" in tree
     let mut handler = File::open("tests/test.jpeg").expect("test.jpeg is stolen");