🔧 Improve typing for MarkdownIt.add_render_rule

[chrisjsewell]

Make the function signtature more explicit

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.14%. Comparing base (bee6d19) to head (a43a87b).
Report is 18 commits behind head on master.

Files with missing lines	Patch %	Lines
markdown_it/main.py	50.00%	1 Missing ⚠️
markdown_it/renderer.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
- Coverage   95.17%   95.14%   -0.03%     
==========================================
  Files          64       64              
  Lines        3437     3443       +6     
==========================================
+ Hits         3271     3276       +5     
- Misses        166      167       +1     

Flag	Coverage Δ
pytests	95.14% <91.66%> (-0.03%)	⬇️

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