chore(js): Adds Registry#apiStability for fine-grained control of bet…

[mbleigh]

I chose to make Registry#apiStability instead of a registry value so that it can be checked synchronously and easily.

The way this works is you can do:

import { assertUnstable } from "@genkit-ai/core";

// inside a feature that is unstable
assertUnstable('beta', 'Option "blah" is a beta feature.');

The error message will automatically be augmented with additional explanation.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs bug: [Docs] Document API stability #1748