feat: make toggle button public in QCollapsible

[tlambert03]

would make #231 easier if the main toggle button were publicly accessible

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b927159) 87.45% compared to head (773e467) 87.29%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
src/superqt/collapsible/_collapsible.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   87.45%   87.29%   -0.17%     
==========================================
  Files          46       46              
  Lines        3355     3336      -19     
==========================================
- Hits         2934     2912      -22     
- Misses        421      424       +3     

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