What problem does this address?

This issue is meant to be a follow-up feature for #42079, suggested by @skorasaurus . After adding support to toggle the Width settings panel for the Button block using theme.json, we should add the same support for the Width setting on the Search block now that there will be a related property intheme.json.

What is your proposed solution?

In the Search block's Edit component, only show the Width setting if true === spacing.width in theme.json, using useSetting() to get the value of spacing.width