Fix/textgrid export complete intervals

[MartinKl]

fixes #283

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.06%. Comparing base (9846c11) to head (7daaf77).

Files	Patch %	Lines
src/exporter/textgrid.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   82.08%   82.06%   -0.02%     
==========================================
  Files          43       43              
  Lines       15285    15290       +5     
  Branches    15285    15290       +5     
==========================================
+ Hits        12546    12548       +2     
- Misses       1043     1046       +3     
  Partials     1696     1696              

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