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-tests/simple.js

@@ -146,3 +146,13 @@ global.MixinFoo = class MixinFoo {
 global.Overloads = class {
   foo() {}
 };
+
+global.InvokeCallback = class {
+  invoke(f) { f(); }
+  callAdd(f) {
+    return f(1, 2);
+  }
+  callRepeat(f) {
+    return f('ab', 4);
+  }
+};