Supports Whisper prompt and prefix

[ocavue]

Feature request

Whisper has two options prompt and prefix defined in DecodingOptions (source code). These two options will be inserted as part of the initial_tokens (source code). It would be good to support them in Transformers.js

Motivation

You can find a detailed description of these two options here: openai/whisper#117 (comment). In short, they can improve the transaction quality for long speak (i.e. larger than 30 seconds).

Your contribution

The Python library transformers has prompt_ids for this feature (source code). Maybe we should just implement it in transformers.js too.