[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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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##           master    #1814   +/-   ##
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  Coverage   87.92%   87.92%           
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  Files          16       16           
  Lines        1375     1375           
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  Hits         1209     1209           
  Misses        166      166           

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