Better way to filter xeus-cpp kernel specs

[jtpio]

#75 adds a script to remove some the kernelspecs, by manually editing the kernels.json from the output dir after jupyter lite build.

This is a bit brittle and it would be great to have a proper way to do that.

Maybe changing the feedstock to have multiple outputs? So we could specify xeus-ccp23 and xeus-c23 in the environment.yml to only get these kernels?

cc @martinRenou @anutosh491