WebVTT generation functionality

[agrathwohl]

Description

A Hypercore output from StreamAnalyserCore will contain time-aligned metadata about an audio recording session, including any transcription text generated in real time (see: AUD-39).

This information is sufficient to produce a WebVTT-formatted string. We should produce a new function that can convert a Hypercore output by StreamAnalyserCore into a WebVTT file. This will enable time-synchronized audio playback with text annotations.

Acceptance Criteria

• The ideal functionality would be provided within our new audio corestore, requiring both the main AudioBuffer feed and an associated StreamAnalyserCore feed to be in the store before the function executes.
• Function should write clipping, silence, and voice activity to the WebVTT output as WebVTT comments.
• Function should write each transcription text object contained in the StreamAnalyserCore as a WebVTT Cue with a cue text payload.

Additional Notes

This function should be exported independently of the class(es) making use of it.