refactor!: config_reader to tmuxp._internal

[tony]

Fixes #894

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5daa1ae) 70.50% compared to head (21f107f) 70.52%.

❗ Current head 21f107f differs from pull request most recent head a631e6f. Consider uploading reports for the commit a631e6f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #897      +/-   ##
==========================================
+ Coverage   70.50%   70.52%   +0.01%     
==========================================
  Files          25       25              
  Lines        1936     1937       +1     
  Branches      407      407              
==========================================
+ Hits         1365     1366       +1     
  Misses        448      448              
  Partials      123      123              

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