Field API format: fix and use WordPress input

[oandregal]

What?

This fixes a bug in the format prop that rendered the date format incorrectly. Additionally, the format is not set like it is in WordPress (via 0-6 numbers, and not via sunday-saturday strings).

Before:

After:

Why?

• It fixes the bug.
• It exposes what WordPress and the components use.

Testing Instructions

• Visit the Storybook for "DataViews > Field Types > Date" and verify the date is rendered properly and changing the format (via storybook controls) works.
• Visit Site Editor > Pages and verify dates are displayed properly.

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[oandregal]

This was the bug.

[oandregal]

This is a time-sensitive issue that fixes an issue for tomorrow's release. I've asked around, but there are no reviewers available, so I'm going to go ahead and merge the fix. Happy to do any follow-ups post-merge. 🙏