Fix selection for component blocks without child fields

[emmatown]

Now you can actually put your text cursor in a component block (and therefore insert content below it or etc.) that doesn't have child fields and the bar on the side will change colour to indicate your selection is inside there.

Try navigating your caret to the carousel block on the docs site:
on this branch: https://keystone-next-docs-git-fix-selection-for-compo-824f49-thinkmill.vercel.app/docs/guides/document-field-demo
before this branch: https://keystone-next-docs-pm1f8fo8c-thinkmill.vercel.app/docs/guides/document-field-demo

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Name	Status	Preview	Updated
keystone-next-docs	✅ Ready (Inspect)	Visit Preview	Oct 20, 2022 at 2:41AM (UTC)

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🦋 Changeset detected

Latest commit: 2079513

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@keystone-6/fields-document	Patch

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[emmatown]

(found a problem)

[emmatown]

The problem was that you would see all of the child fields after the form (yes, that could be desirable behavior but it would need to be presented well which this was not), I've fixed that by reverting the change for edit mode. So selection won't work right when in the edit mode of a component block, I think that's good enough since people can click done and then it'll work right again.