[ENH] Add stimulus and annotation entities

[neuromechanist]

stimulus and annotation entities are necessary to distinguish various stimulus files and possible different annotations per each stimulus file.

A sample use-case of these entities is presented in bids-standard/bids-examples#433.

Both entities are being discussed in #153, and this Google Doc.
The stim- entity was suggested first in #751.
The annot- entity might be of use in #1771 as well.

This pull request would likely require a minor version change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.92%. Comparing base (0950f6d) to head (cee0301).

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[yarikoptic]

A parallel to investigate here is _events.tsv files, i.e. those could also be created/annotated across various dimensions. ATM they would just "flatten" them into this single file with some column potentially disentangling those dimensions for different annotations (e.g. low level properties of visual stimuli vs higher level cognitive concepts vs responses). So it feels to me it might be applicable there too.

But with that it somewhat begs me to think about some possible alternative name/concept but I find none and there be none ;)

[yarikoptic]

Since we already ran into discussion either desc could be in effect used as desc (and we have example in bids of _desc-autoannot), it would be valuable to describe here the difference from desc, e.g. annot is more specialized in providing annotations to the stimuli/events whenever desc is more about processing of the data. So _annot-chatgpt would be preferable over _desc-chatgpt as would symbolize annotation over some other chatgpt driven processing (e.g. prompted chatgpt for some analysis of data/stimuli)