Proposed updates to availability values

[domenic]

We've received feedback in webmachinelearning/translation-api#12 and webmachinelearning/prompt-api#18 that the values "no", "readily", and "after-download" for the availability() functions might be unclear to non-native speakers.

At some point these were answers to the question canCreateXYZ(), at which point I think those names were pretty good. But now that the function is named availability() they are not as good.

We propose the new names "unavailable", "available", and "downloadable".

In addition, we propose adding a fourth state, "downloading". In the current proposal, if a model is currently downloading we'll return "after-download". But the desired user experience for if the download is not yet started, vs. if the download is already started, could be pretty different. A site might want to go ahead and create a summarizer/etc. if the download is already started, whereas they might want to prompt the user to check if the download has not yet started.

Feedback on this is welcome.