Add IDBKeyRange.forPrefix(key)

[inexorabletash]

See https://groups.google.com/a/chromium.org/forum/#!topic/chromium-html5/xBl5yhH41UY

Polyfill: https://gist.github.com/inexorabletash/5462871

[inexorabletash]

TPAC 2019 Web Apps Indexed DB triage notes:

We think this should be tackled to make IDB usable out of the box. Behavior should be specified, lower/upper on the range should return real values... what we do for arrays still needs defining.

[inexorabletash]

Example use case: https://stackoverflow.com/questions/76657707/how-to-search-for-a-single-column-when-using-compound-indexes-in-indexeddb