Implement SupportedPropertyNames methods

[frewsxcv]

#7254 introduces a method on some DOM types called SupportedPropertyNames that returns all the 'supported property names' as per their respective interface. That PR only implements HTMLCollectionMethods::SupportedPropertyNames (I didn't have enough time to complete the rest). The other ones need to be implemented.

[frewsxcv]

At the time of writing, there is one impl remaining to be implemented:

servo/components/script/dom/document.rs

Lines 2529 to 2533 in 53aca4b

	// https://html.spec.whatwg.org/multipage/#dom-tree-accessors:supported-property-names
	fn SupportedPropertyNames(&self) -> Vec<DOMString> {
	// FIXME: unimplemented (https://github.com/servo/servo/issues/7273)
	vec![]
	}

[pshaughn]

A few years later, the above comment continues to be effectively true. (Plugin and PluginArray also have stubs, but that's correct for a plugin-less world.)