Add fenced code block support

[fingolfin]

Document Markdown-style fenced code blocks, add regression
coverage, and convert them into GAPDoc listings.

Resolves #184

Co-authored-by: Codex codex@openai.com

[codecov]

Codecov Report

❌ Patch coverage is 97.27273% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.43%. Comparing base (9dea6e7) to head (0f05ab4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Markdown.gi	94.73%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   84.82%   85.43%   +0.60%     
==========================================
  Files          13       13              
  Lines        2280     2389     +109     
==========================================
+ Hits         1934     2041     +107     
- Misses        346      348       +2     

Files with missing lines	Coverage Δ
gap/Parser.gi	82.96% <100.00%> (+1.31%)	⬆️
gap/Markdown.gi	96.70% <94.73%> (-0.90%)	⬇️

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