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>
