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`.

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

pyscriptjs/tests/py-unit/js.py

@@ -4,3 +4,4 @@ from unittest.mock import Mock
 document = Mock()
 console = Mock()
 setTimeout = Mock()
+Object = Mock()