Fixes #14339

### Additions

Adds `InfoDict` type to improve the typing of `info()` result.

Adds missing `"subclasses"` to `info_fields` list (these were added to
the field list in #11486 but we
forgot to update `info_fields` variable at the time) - the newly added
`InfoDict` type will ensure that this won't happen again.

Adds `InspectorHookData` dataclass which is passed to the MIME hooks
which now should expect a single argument. Having a single dataclass
argument enables us to deprecate individual fields, or add new fields
without breaking the existing hooks. The old hooks will still work (if
any are out there since this mechanism got just added in the previous
point version).

### Deletions

A comment over `info_fields` gets deleted:
- Contrarily to the comment (which is getting deleted in this PR),
`info_fields` were not defining the order of display since at least 2015
(#7903 - I did not feel the need
to go further in the history to find when exactly it happened).
- Also contrarily to this comment, current Jupyter messaging spec does
not define the contents of `info_fields` (I guess this was lost during
IPython/Jupyter split), but the newly added `InfoDict` at least properly
annotates their type (if you know where I can find the old IPython
messaging spec with the descriptions I can add these as doc comments).

Unused `cast_unicode` import gets deleted. If someone imported it from
here... well they really should not have.

### Deprecations

- mime hooks taking two arguments (`obj, info`)