Support dictionaries in unions

[nox]

We currently can't do (FooInit or boolean) where FooInit is a dictionary in a WebIDL interface.

[KiChjang]

Related to #10605.

[nox]

So the problem is that for a dictionary Foo, the generated Foo::new function do not let callers choose whether they want to report exceptions or not.

I think the FromJSValConvertible trait should be changed to return Result<T, Cow<str>>, where callers can use that string to raise a TypeError if needed.

[jdm]

With the following added to a WebIDL file:

dictionary EventListenerOptions {
  boolean capture = false;
};

void foo(optional (EventListenerOptions or boolean) options);

we get

TypeError: Can't handle EventListenerOptions (Wrapper) in unions yet

The following patch:

diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index f531937..6138abb 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -4028,6 +4028,9 @@ def getUnionTypeTemplateVars(type, descriptorProvider):
     elif type.isEnum():
         name = type.inner.identifier.name
         typeName = name
+    elif type.isDictionary():
+        name = type.name
+        typeName = name
     elif type.isSequence() or type.isMozMap():
         name = type.name
         inner = getUnionTypeTemplateVars(innerContainerType(type), descriptorProvider)

makes this error appear instead:

TypeError: Can't handle dictionaries when failureCode is not None

[Ms2ger]

I'm having a look.