Feat: GST sentence

[elpiel]

Based on #121 to fix a merge conflict.
Thank you @zhrbrk for working on this sentence, sorry it took me some time to review your PR I got to a point and then left the PR review unfinished.

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.11%. Comparing base (6bfcd5a) to head (879b3f4).

Files	Patch %	Lines
src/sentences/gst.rs	87.50%	3 Missing ⚠️
src/parser.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   77.96%   78.11%   +0.14%     
==========================================
  Files          35       36       +1     
  Lines        1307     1334      +27     
==========================================
+ Hits         1019     1042      +23     
- Misses        288      292       +4     

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