WhisperX implementation

[JonasCz]

This adds WhisperX, useful to me because it's faster and has word-level timestamps.

(Word level timestamps are used here in order to "regroup" subtitle lines more logically, e.g. each sentence gets its own subtitle line if possible, and otherwise lines are split in a more sensible way instead of the pretty "random" output of base Whisper).

This probably:

Closes (sort of) #53 Progress #58 (WhisperX supports diarization, although this doesn't do that)

In order to use it, you have to:

• Add WhisperX to transcription-api/requirements.txt instead of faster-whisper
• Replace call to FasterWhisperBackend in transcribe.py with WhisperxBackend

The code is quite messy, it's "minimal effort" adapted from one of my other projects.