Revamp the API surface

There are two major motivations for this change:

* Splitting translation and language detection into separate APIs, to reflect what we've learned from prototyping.

* Aligning better with other built-in API proposals, including future ones, by using shared patterns.

This notably removes translation from an unknown source language, closing #1. It also adds AbortSignals and destroy() methods.

This also removes the tentative proposal for language tag handling, instead pointing to discussions in #11.

Author: domenic