script_bindings: Assert that serializable/transferable types have accurate WebIDL annotations (#38615)

These changes add compile-time assertions that:
* any type that implements the Serializable/Transferable trait has a
`[Serializable]` or `[Transferable]` annotation in the interface WebIDL
* any WebIDL interface with the `[Serializable]` or `[Transferable]`
annotation implements the corresponding trait

This is useful because it means that WebIDL definitions will be less
confusing if you're trying to figure out whether Servo supports
serializing/transferring a particular interface type. It also makes
fixing #21715 in the future a little bit easier, because the annotations
will remain up to date.

Testing: compile-time only; no point in writing tests for this since it
involves webidl codegen.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/structuredclone.rs

@@ -0,0 +1,17 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+/// A marker to ensure that the `[Serializable]` attribute is present on
+/// types that can be serialized. This trait should not be implemented manually.
+pub trait MarkedAsSerializableInIdl {
+    /// Used to define compile-time assertions about the type implementing this trait.
+    fn assert_serializable();
+}
+
+/// A marker to ensure that the `[Transferable]` attribute is present on
+/// types that can be transferred. This trait should not be implemented manually.
+pub trait MarkedAsTransferableInIdl {
+    /// Used to define compile-time assertions about the type implementing this trait.
+    fn assert_transferable();
+}