Remove/replace `codemirror_mode` field?

This is leaking implementation detail of old notebook version and not used by modern frontends. We could have some standard `syntax_highlighting` field, possibly including a MIME type in there + some options.

Start by reviewing how the notebook format currently represents codemirror_mode and how modern frontends consume that field. Clarify whether it should be removed or replaced with syntax_highlighting, including MIME type and options, then establish the format and compatibility checks that would define completion.

Written by the indexing model from the issue text.