Add @when decorator (#1428)

* Add new _event_handling.py file with @when decorator

* @when decorate is in pyscript package namespace/_all__

* Write tests in new test_event_handling.py

* Add docs for @when decorator

------------

Co-authored-by: Mariana Meireles <marian.meireles@gmail.com>

pyscriptjs/tests/integration/test_01_basic.py

@@ -316,6 +316,7 @@ class TestBasic(PyScriptTest):
         )
         btn = self.page.wait_for_selector("button")
         btn.click()
+        self.wait_for_console("hello world!")
         assert self.console.log.lines[-1] == "hello world!"
         assert self.console.error.lines == []