* 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
This PR does two things:
1. introduce the first minimal version of a Plugin system. Plugins are subclasses of the Plugin class, and pyscript will call the relevant methods/hooks at the right time. Currently, I have added only the minimal sets of hooks which were needed for this PR.
2. Implement <py-terminal> as a plugin.