Split pyscript into multiple files (#1338)

In the future this should help us leak fewer names into the pyscript
namespace.

Rather than assigning to the pyscript module from JavaScript, we
mount a separate private JS module with the extra names needed by
PyScript. I moved a bit more interpeter intialization into
remote_interpreter.

I added a deprecation warning for `pyscript.js`: the proper way to
access `js` is `import js`.

---------

Co-authored-by: Antonio Cuni <anto.cuni@gmail.com>

pyscriptjs/tests/py-unit/_pyscript_js.py

@@ -0,0 +1,4 @@
+from unittest.mock import Mock
+
+showWarning = Mock()
+define_custom_element = Mock()