FEAT: Add extend grammar

[ViciousDoormat]

extend a grammar with a rule created from an AST

PR requested by @THinnerichs in Herb-AI/HerbSearch.jl#126 (comment)_

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.73%. Comparing base (f1ba20b) to head (c4f0cc4).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   45.83%   46.73%   +0.90%     
==========================================
  Files           7        7              
  Lines         456      460       +4     
==========================================
+ Hits          209      215       +6     
+ Misses        247      245       -2     

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