Add event utils

snippets/off.md

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+### off
+
+Removes an event listener from an element.
+
+Use `EventTarget.removeEventListener()` to remove an event listener to an element.
+
+```js
+const off = (el, evt, fn, opts = false) => el.removeEventListener(evt, fn, opts);
+```
+
+```js
+// See the `on` snippet.
+const fn = () => console.log('!');
+const ref = on(document.body, 'click', fn, { target: 'p' });
+off(document.body, 'click', ref); // no longer logs '!' upon clicking a `p` el child of the body
+```