refactor!: `config_reader` to `tmuxp._internal`

[tony]

Fixes #894

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.