enables functionality of holding SHIFT to draw circle or square

[zoccoler]

This implementation is not done via matplotlib itself because it does not work if the canvas is used inside napari (napari may overwrite some default hotkeys)

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 89.21%. Comparing base (8639d09) to head (44b8922).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/biaplotter/selectors.py	40.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   90.80%   89.21%   -1.60%     
==========================================
  Files           7        7              
  Lines         631      649      +18     
==========================================
+ Hits          573      579       +6     
- Misses         58       70      +12     

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