Add messaging docs for optional features and kernel subshells

[ianthomas23]

This adds messaging documentation for optional features JEP92 and kernel subshells JEP91.

Summary of changes:

• Optional features JEP92:

  • Return optional supported_features in kernel_info_reply.
  • Deprecate use of debugger in kernel_info_reply as this is replaced by supported_features=['debugger']. It will have to be a long deprecation period whilst other libraries in the Jupyter ecosystem adopt use of supported_features.

• Kernel subshells JEP91:

  • Return supported_features=['kernel subshells'] in kernel_info_reply.
  • New control request/reply messages for create, delete and list subshells.
  • Add optional subshell_id to message header.

The version added/changed for all of these is 5.5. I am unsure of the protocol around official releases here.

[davidbrochart]

Thanks @ianthomas23.
Just one minor comment: should we use capital letters for "ID"? I see we use both "ID" and "id" in the current documentation, but I would prefer "ID".

[ianthomas23]

Just one minor comment: should we use capital letters for "ID"? I see we use both "ID" and "id" in the current documentation, but I would prefer "ID".

Good idea. I've switched to using "ID" throughout the messaging page for consistency.