`^js` tag

[shaunlebron]

question came up here: lilactown/helix#104 (comment)

discussed with @thheller here: https://clojurians.slack.com/archives/C03S1L9DN/p1664298527478269

^js can effectively be used instead of ^js/React.Element. The extra type info is for readability, but shadow-cljs for example actually throws it away. I think Closure conservatively refuses to mangle recognized property names regardless of the object type.

Looking at both cljs and shadow-cljs codebases, here’s how the ^js tag affects externs. I’ll probably put this on cljs api site soon since there’s much confusion about what actually happens:

	added to cljs externs	added to shadow-cljs externs
^js =>	Object.*	ShadowJS.prototype.*
^js/React.Element =>	React.Element.prototype.*	ShadowJS.prototype.*