feat dynamic event handler

[tanhauhau]

Fix #3040

Note: I've taken this chance to move the EventHandler rendering logic into render_dom/ folder.

[Rich-Harris]

Excellent, this is a much-needed bugfix. I was wondering about an alternative approach — given the same input...

<script>
	let clickHandler;
	function updateHandler2(){
		clickHandler = () => number = 2;
	}
</script>

<button on:click={clickHandler}>click</button>

...what if we just wrapper the listen argument in a function, only in cases where the value is dynamic?

function create_fragment(ctx) {
	var button, dispose;

	return {
		c() {
			button = element("button");
			button.textContent = "click";
-			dispose = listen(button, "click", ctx.clickHandler);
+			dispose = listen(button, "click", function(e) {
+				ctx.clickHandler.call(this, e);
+			});
		},

It would require maintain_context to be true for that block, but I think it would result in better performance in pathological cases (where addEventListener and removeEventListener could be getting called very frequently) and slightly less code. Thoughts?

[tanhauhau]

This is exactly the workaround I suggested for now.

Yea i think it's a good idea, why didnt i think of it in the first place?

[tanhauhau]

i've make sure it works for component event handler as well.

[Rich-Harris]

sweet. thanks!