split pyjs up in pyjs and pyjs_core

[DerThorsten]

pyjs implements a lot of its functionality in pure Python on top of the C++ Code. But since pyjs is an embedded Python module (ie one which lives directly in the code instead of a side package) the pure Python code is added via py::eval("some code") instead of files.
This is ugly and hard to maintain, therefore pyjs used some hacks to create the py::eval("some code") statements from actual files with Python code. But this hack had some scoping issues and is ugly and not nice to maintain and explain.
Therefore this PR moves the pure Python code to a dedicated top-level package called pyjs. The old pyjs embedded package is renamed to pyjs_core.
From a user / API perspective, nothing changes since we can do from pyjs_core import * within pyjs

[DerThorsten]

needs a new release of pyjs-code-runner with emscripten-forge/pyjs-code-runner#14 ( ATM we install pyjs-code-runner from main by hand in the CI in this PR)