refactor!: Move _types to _internal/types

[tony]

Fixes #899

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8282ecd) 70.52% compared to head (5b1f940) 70.71%.

Files	Patch %	Lines
src/tmuxp/plugin.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #900      +/-   ##
==========================================
+ Coverage   70.52%   70.71%   +0.19%     
==========================================
  Files          25       26       +1     
  Lines        1937     1950      +13     
  Branches      407      407              
==========================================
+ Hits         1366     1379      +13     
  Misses        448      448              
  Partials      123      123              

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