Normative: Add null type, and allow it as a return type (excl. unions)

[ExE-Boss]

This complements #906 and makes it possible to specify the legacy PluginArray and MediaTypeArray APIs without using a nullable object? type: whatwg/html#6296 (comment)

 [Exposed=Window]
 interface PluginArray {
 	undefined refresh();
 	readonly attribute unsigned long length;
-	object? item(unsigned long index);
-	object? namedItem(DOMString name);
+	null item(unsigned long index);
+	null namedItem(DOMString name);
 };
 
 [Exposed=Window]
 interface MimeTypeArray {
 	readonly attribute unsigned long length;
-	object? item(unsigned long index);
-	object? namedItem(DOMString name);
+	null item(unsigned long index);
+	null namedItem(DOMString name);
 };

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Unlike #866, this keeps nullable types for now.

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[annevk]

I think we need a more compelling use case to change IDL.

[domenic]

Folks generally agree this makes sense, but isn't needed until this comes up again a couple of times. (Especially since the HTML plugin example no longer works.) Something like this might fall out of a larger project to straighten out unions/typedefs, and the relationship between ?, null, and undefined.