Extern returning in create.GetInstance

[EstebenR]

GetInstance was returning type Instance for any singleton roblox type passed to Fluid.PropertiesFor<> because they count as extern.

This change allows for creating components extending roblox types comfortably like so

return function(props: Fluid.PropertiesFor<TextLabel> & { HoverColor: Color3 })
	return Fluid.create("TextLabel")({
		Text = "Test",
		BackgroundTransparency = props.BackgroundTransparency or 1,

		Fluid.create("ImageLabel")({}),
	})
end

Previously this would have thrown a type error, as BackgroundTransparency was not present in type Instance.