Resolves pyscript#1313 and pyscript#1314. On top of pyscript#1318.
The point of these tests is to define the execution order of Tasks
that are scheduled in <py-script> tags: first all the py-script tags
are executed and their related lifecycle events. Once all of this
is done, we schedule any enqueued tasks.
To delay the execution of these tasks, we use a custom event loop for
pyExec with this defer behavior. Until schedule_deferred_tasks is called,
we defer tasks started by user code. schedule_deferred_tasks starts all
deferred user tasks and switches to immediately scheduling any further
user tasks.
* Add beforePyScriptExec, afterPyScriptExec lifecycle hooks
* Add stdiodirector plugin for `output`, `stderr` attributes of py-script tag
* Add docs on `output` and `stderr` attributes of py-script tag
* Tests
* Removed output deprecation warning for `output` attribute
* Add createSingularWarning(), with createDeprecationWarning as alias
