Report source positions for Markdown errors

[fingolfin]

Preserve source locations while parsing and rendering markdown so
unmatched backticks and emphasis markers report the offending file
and line instead of raising a bare Error.

AI assistance: Codex implemented the change and added regression
coverage for markdown syntax errors.

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

[codecov]

Codecov Report

❌ Patch coverage is 95.80838% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.14%. Comparing base (37976b8) to head (3b1375b).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Markdown.gi	91.80%	5 Missing ⚠️
gap/Parser.gi	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
+ Coverage   92.94%   93.14%   +0.19%     
==========================================
  Files          13       13              
  Lines        2779     2918     +139     
==========================================
+ Hits         2583     2718     +135     
- Misses        196      200       +4     

Files with missing lines	Coverage Δ
gap/DocumentationTree.gi	96.07% <100.00%> (-0.01%)	⬇️
gap/ToolFunctions.gd	100.00% <100.00%> (ø)
gap/ToolFunctions.gi	94.57% <100.00%> (+1.10%)	⬆️
gap/Parser.gi	96.64% <91.30%> (-0.03%)	⬇️
gap/Markdown.gi	97.22% <91.80%> (-0.55%)	⬇️

... and 2 files with indirect coverage changes

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