WIP: add type hints to mne/_fiff/ module

[drammock]

opening as draft PR so folks can take a look and tell me what I did wrong, before I move on to doing the remaining files in mne/_fiff/

[larsoner]

I think any change to typing should carry a ty change that allowlists (or un-blocklists) the files if possible. I think at the end of the day we can lean on ty plus our existing unit tests (including the test_docstring_parameters to make sure things make sense and are correctly applied

[drammock]

I think any change to typing should carry a ty change that allowlists (or un-blocklists) the files if possible.

Oops. Totally agree and meant to do that; had been locally running ty manually on the changed files.