Components: Assess stabilization of __unstableInputWidth

[mirka]

What problem does this address?

Several components (e.g. InputControl and family) have a __unstableInputWidth prop, which has been experimental for very long. This prop is required for labelPosition="edge" to work (labelPosition is a stable prop).

SelectControl has a labelPosition prop, but the "edge" option cannot be utilized correctly because the __unstableInputWidth prop is not exposed.

It's also important to note that the labelPosition prop is barely used, aside from "top" which is the default. There are only two use cases in the Gutenberg app (both are "edge"). There is not a strong need for this feature.

__unstableInputWidth itself, independent of labelPosition, is used a little bit more. But still not a lot.

What is your proposed solution?

Assess if we really want to stabilize this, or if we would prefer consumers to do custom layouts in a more modular way.