Scdoc: new parser

[masatake]

reference: https://git.sr.ht/~sircmpwn/scdoc/tree/master/item/scdoc.5.scd

[codecov]

Codecov Report

Attention: Patch coverage is 6.25000% with 45 lines in your changes missing coverage. Please review.

Project coverage is 85.83%. Comparing base (d0368a8) to head (64d21e8).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
optlib/scdoc.c	0.00%	45 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4244      +/-   ##
==========================================
- Coverage   85.89%   85.83%   -0.07%     
==========================================
  Files         243      244       +1     
  Lines       62847    62895      +48     
==========================================
+ Hits        53985    53988       +3     
- Misses       8862     8907      +45     

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