Allow [up-emit] on buttons

[triskweline]

Discussed in #411

^{Originally posted by outcassed August 9, 2022}
Would it be possible to have unpoly either honor up-emit on <button>s or expose the up-emit click event handler so that it can be easily extended?

--- unpoly.js
+++ unpoly-patch.js
@@ -11642,6 +11642,7 @@
     //    abort = -> up.emit(signal, 'abort')
     //    [abort, signal]
     on('up:click', 'a[up-emit]', executeEmitAttr);
+    on('up:click', 'button[up-emit]', executeEmitAttr);
     on('up:framework:reset', reset);
     return {
         on,
@@ -11654,7 +11655,8 @@
         halt,
         isUnmodified,
         fork,
-        keyModifiers
+        keyModifiers,
+        executeEmitAttr
     };
 })();
 up.on = up.event.on;
```</div>

[triskweline]

This should also work on an input[type=button].

We could just drop the element type from the selector and emit events on anything with an [up-emit] attribute. This is how we already do it for [up-dismiss] and [up-accept].

[alexandergitter]

Here is another example: I have a table where I would like to be able and emit an event when a row is clicked. Currently I could add an <a> in every <td>, or use <a>s for the rows and set display: table-row on them.
At the moment I use Alpine.js with an x-on:click attribute, that in turn calls an Unpoly function. It would be nice to be able to do this with Unpoly directly.