Add support for SRT subtitles with tests and CLI updates

[GitBib]

Description

This pull request introduces SRT subtitle format support to the project. Key changes include:

• Implemented SRT format detection and conversion in the Subtitle class.
• Added extensive test coverage for SRT parsing, export functionality, and edge cases.
• Enhanced the CLI to process SRT files for export and dialogue extraction.
• Refactored the test suite to replace os with pathlib for improved file handling.
• Updated test fixtures by adding a sample SRT file for testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.38%. Comparing base (d704a4d) to head (af29cf1).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   99.27%   99.38%   +0.11%     
==========================================
  Files           5        5              
  Lines         138      163      +25     
==========================================
+ Hits          137      162      +25     
  Misses          1        1              

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