Refactor @when and add Event (#2239)

* Add two unit tests for illustrative purposes.

* Radical simplification of @when, more tests and some minor refactoring.
Handle ElementCollections, tests for ElementCollection, make serve for running tests locally.

* Skip flakey Pyodide in worker test (it works 50/50 and appears to be a timing issue).

* Ensure onFOO relates to an underlying FOO event in an Element.

* Minor comment cleanup.

* Add async test for Event listeners.

* Handlers no longer require an event parameter.

* Add tests for async handling via when.

* Docstring cleanup.

* Refactor onFOO to on_FOO.

* Minor typo tidy ups.

* Use correct check for MicroPython.

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Co-authored-by: Andrea Giammarchi <andrea.giammarchi@gmail.com>

core/tests/python/index.html

@@ -62,6 +62,7 @@
             <button id="a-test-button">I'm a button to be clicked</button>
             <button>I'm another button you can click</button>
             <button id="a-third-button">2 is better than 3 :)</button>
+            <button id="another-test-button">I'm another button to be clicked</button>
 
             <div id="element-append-tests"></div>
             <p class="collection"></p>