Narrow down the `IKernel` interface?

The current IKernel interface currently defines a couple of methods such as executeRequest and completeRequest:

https://github.com/jupyterlite/jupyterlite/blob/209d48100d598014df7e6c17993b3fdca279c984/packages/kernel/src/tokens.ts#L45

Which are then called in BaseKernel here:

https://github.com/jupyterlite/jupyterlite/blob/209d48100d598014df7e6c17993b3fdca279c984/packages/kernel/src/kernel.ts#L545

This made it convenient to implement the first JavaScript and Python kernels extending BaseKernel, as it required to implement just a couple of methods to get a basic kernel to work.

However in the case of xeus based kernels, these are already handled and implemented. Having the TypeScript counterpart sounds redundant and not necessary.

We should then check whether we would like to narrow the IKernel down, for example to just the handleMessage method. And give full-control to kernel authors as to how they handle the messages.

cc @DerThorsten since we briefly discussed this

[martinRenou]

It will also be handy for cleaning the pyolite kernel as well, as the worker is responsible for dispatching the message to the right requests.

[jtpio]

Fixed by #355