refactor: simplify ffmpeg backend

[escaped]

• simplify the timecode extraction
• add type hints to the backend

[coveralls]

Pull Request Test Coverage Report for Build 319266910

• 33 of 34 (97.06%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 91.484%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
video_encoding/backends/ffmpeg.py	30	31	96.77%
<!--	Total:	33	34	97.06%	-->

Totals
Change from base Build 318800913:	0.6%
Covered Lines:	293
Relevant Lines:	312

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💛 - Coveralls