Editorial: Use named records for DateTimeFormat format records

[anba]

This PR aims to restructure Intl.DateTimeFormat Constructor - Internal Slots to use named Record types instead of ad-hoc records. In the process of this clean-up, some editorial bugs were found and fixed, see the commit messages for details.

In addition to providing a more type-safe approach when compared to using ad-hoc records, these changes should also help with the Temporal integration, see https://tc39.es/proposal-temporal/#sec-temporal-intl.

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The PR applies on top of #822.

[anba]

Fixed all review comments and rebased the commits to apply cleanly on tip.

[anba]

Rebased again.

Do we still wait for another review or can we land these changes?

[ben-allen]

@anba I've rebased this onto the current main on my own fork: https://github.com/ben-allen/ecma402/tree/anba-826-working-branch. If you pull it in, I can merge.