feat: add a command for promoting components to GA

We often move components from preview to GA (pre-1.0 to 1.0).

Create a command that makes the necessary changes. This includes:
- migrating the README to say that the component is stable/GA
- updating the component in `.repo-metadata.full.json` from "preview" to "stable"
- potentially tell release-please to make the next version stable (I'm quite sure how this would work...)

This is all for now, but in the future there could be more steps, so it'd be nice to have them automated.