Split javascript and python source files

[echarles]

This PR does not change the functionality, it just splits the javascript and python to make the source code more readable.

It also ensure the extension can be built with npm.

[github-actions]

👈 Launch a Binder on branch datalayer/jupyter-server-nbmodel/fix%2Fnpm

[krassowski]

Just a note that I would recommend releasing this in a new minor version because users will need manually update jupyter-server-nbmodel:notebook-cell-executor if they were disabling this plugin as it is now called @datalayer/jupyter-server-nbmodel:notebook-cell-executor (though it does not need to be - you could publish under @datalayer on npm without renaming the plugin).