libservo: Stop using script_traits in the embedding layer (#35185)

Many types used directly in the `libservo` API are in the
`script_traits` crate, which was created to break circular dependencies.
Move all API exposed types to `embedder_traits` which now contains types
exposed via the `libservo` embedding API. Also expose these at the root
of the `libservo` `servo` crate so that the API won't break when they
move around in the future.

The idea with `embedder_traits` in the future is that it contains types
that are available throughout servo because they are used in the
embedding API and thus should have minimal dependencies on other Servo
crates (a bit like `base`).

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

ports/servoshell/egl/android/resources.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 use std::path::PathBuf;
 
-use servo::embedder_traits::resources::{Resource, ResourceReaderMethods};
+use servo::resources::{Resource, ResourceReaderMethods};
 
 pub(crate) struct ResourceReaderInstance;