Implement support for WebIDL Callback types

This commit adds support for the WebIDL `Callback` type by translating all
callbacks to the `js_sys::Function` type. This will enable passing raw JS values
into callbacks as well as Rust valus using the `Closure` type.

This commit doesn't currently implement "callback interfaces" in WebIDL, that's
left for a follow-up commit.

crates/webidl/src/first_pass.rs

@@ -34,6 +34,7 @@ pub(crate) struct FirstPassRecord<'src> {
     pub(crate) namespaces: BTreeMap<&'src str, NamespaceData<'src>>,
     pub(crate) includes: BTreeMap<&'src str, BTreeSet<&'src str>>,
     pub(crate) dictionaries: BTreeMap<&'src str, DictionaryData<'src>>,
+    pub(crate) callbacks: BTreeSet<&'src str>,
 }
 
 /// We need to collect interface data during the first pass, to be used later.
@@ -135,12 +136,9 @@ impl<'src> FirstPass<'src, ()> for weedle::Definition<'src> {
             Namespace(namespace) => namespace.first_pass(record, ()),
             PartialNamespace(namespace) => namespace.first_pass(record, ()),
             Typedef(typedef) => typedef.first_pass(record, ()),
+            Callback(callback) => callback.first_pass(record, ()),
 
             Implements(_) => Ok(()),
-            Callback(..) => {
-                warn!("Unsupported WebIDL Callback definition: {:?}", self);
-                Ok(())
-            }
             CallbackInterface(..) => {
                 warn!("Unsupported WebIDL CallbackInterface definition: {:?}", self);
                 Ok(())
@@ -685,6 +683,13 @@ impl<'src> FirstPass<'src, &'src str> for weedle::namespace::OperationNamespaceM
     }
 }
 
+impl<'src> FirstPass<'src, ()> for weedle::CallbackDefinition<'src> {
+    fn first_pass(&'src self, record: &mut FirstPassRecord<'src>, _: ()) -> Result<()> {
+        record.callbacks.insert(self.identifier.0);
+        Ok(())
+    }
+}
+
 impl<'a> FirstPassRecord<'a> {
     pub fn all_superclasses<'me>(&'me self, interface: &str)
         -> impl Iterator<Item = String> + 'me