Improve type of PropertiesFor

[checkraisefold]

This makes the PropertiesFor type function return a more accurate type to what is actually accepted by fluid's evaluator by improving children in the table along with allowing the entire properties type to be nested.

For example, this code will no longer throw a type error:

return function(props: fluid.PropertiesFor<"ImageButton">)
	return create "ImageButton" {
		props,
	}
end