Move more bindings code to script_bindings (#35578)

* Move JSContext wrapper to script_bindings.

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

* Move webidl constant bindings to script_bindings.

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

* Move CanGc to script_bindings.

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

* Move Dom<T> and Root<T> types to script_bindings.

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

* Formatting.

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

* Extra docs for new traits.

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

* Fix clippy warnings.

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

---------

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

components/script_bindings/script_runtime.rs

@@ -3,6 +3,35 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::cell::Cell;
+use std::marker::PhantomData;
+use std::ops::Deref;
+
+use js::jsapi::JSContext as RawJSContext;
+
+#[derive(Clone, Copy)]
+#[repr(transparent)]
+pub struct JSContext(*mut RawJSContext);
+
+#[allow(unsafe_code)]
+impl JSContext {
+    /// Create a new [`JSContext`] object from the given raw pointer.
+    ///
+    /// # Safety
+    ///
+    /// The `RawJSContext` argument must point to a valid `RawJSContext` in memory.
+    pub unsafe fn from_ptr(raw_js_context: *mut RawJSContext) -> Self {
+        JSContext(raw_js_context)
+    }
+}
+
+#[allow(unsafe_code)]
+impl Deref for JSContext {
+    type Target = *mut RawJSContext;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
 
 thread_local!(
     static THREAD_ACTIVE: Cell<bool> = const { Cell::new(true) };
@@ -15,3 +44,19 @@ pub fn runtime_is_alive() -> bool {
 pub fn mark_runtime_dead() {
     THREAD_ACTIVE.with(|t| t.set(false));
 }
+
+#[derive(Clone, Copy, Debug)]
+/// A compile-time marker that there are operations that could trigger a JS garbage collection
+/// operation within the current stack frame. It is trivially copyable, so it should be passed
+/// as a function argument and reused when calling other functions whenever possible. Since it
+/// is only meaningful within the current stack frame, it is impossible to move it to a different
+/// thread or into a task that will execute asynchronously.
+pub struct CanGc(PhantomData<*mut ()>);
+
+impl CanGc {
+    /// Create a new CanGc value, representing that a GC operation is possible within the
+    /// current stack frame.
+    pub fn note() -> CanGc {
+        CanGc(PhantomData)
+    }
+}