Refactor parser command splitting

[fingolfin]

Add a dedicated command/argument splitter and use it in Scan_for_AutoDoc_Part.

Refactor InstallMethod parsing into local helper functions and remove unused parser locals.

Add misc.tst coverage for plain-text command splitting.

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

[codecov]

Codecov Report

❌ Patch coverage is 93.50649% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.97%. Comparing base (22933a2) to head (5029948).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Parser.gi	93.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
+ Coverage   84.72%   84.97%   +0.25%     
==========================================
  Files          13       13              
  Lines        2200     2217      +17     
==========================================
+ Hits         1864     1884      +20     
+ Misses        336      333       -3     

Files with missing lines	Coverage Δ
gap/Parser.gi	82.47% <93.50%> (+0.87%)	⬆️

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