Details block: use summary content as default label

[dougwollison]

What?

Allows the Details block to use it's summary attribute as the block label by default, similar to the Heading block.

Why?

The Details block has a viable attribute to use as the default label, and in use cases where there are dozens on a page, it makes the List View more usable.

How?

Defines custom __experimentalLabel handler to the Details block. Unsure if the accessibility label should be prefixed with "Details. " or not, tried to match the approach of the Heading block.

Testing Instructions

1. Open the editor
2. Add a Details block, give it a summary value
3. Open List View, confirm summary is being used as the label

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Co-authored-by: dougwollison <[email protected]>

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[talldan]

Code looks good to me, and tests well. Thanks for the contribution!