[Emscripten] generate NPM package content

[SylvainCorlay]

Most of the content of the xeus-*-kernel JupyterLite kernels is duplicated. We should presumably be able to

• Include a (pure-js) worker.js in the xeus repository. It does not make sense to make a standalone package because any change to the wasm interface would prescribe a change to that file, and it is just 50 lines long.
• Generate the content of the npm and python package including xeus_server_kernel.ts.
• The content of index.ts could also be generated from the kernelspec which could have a special cmake install rule when building for wasm.

[SylvainCorlay]

The question is how to make that work without things being put together with duck tape...