feat: add timezone support on date fields

[paulpopus]

Adds support for timezone selection on date fields.

Summary

• New timezone config option on date fields
• New timezone.supportedTimezones in global admin config to allow customisation of available timezones
  • timezone.defaultTimezone sets the default selection
• formatOptions has been moved to its own file
• supportedTimezones is an array of possible timezones with IANA values exported from payload
• siblingField is now threaded through to each field for direct sibling config access
• generates a new type for SupportedTimezones which is then reused across fields

Configuration

All date fields now accept timezone: true to enable this feature, which will inject a new field into the configuration using the date field's name to construct the name for the timezone column. So publishingDate will have publishingDate_timezone as an accompanying column. This new field is inserted during config sanitisation.

Example:

{
  type: 'date',
  name: 'date',
  timezone: true,
}

In the future we can turn this to accept a config object as well if needed.

Dates continue to be stored in UTC, this will help maintain dates without needing a migration and it makes it easier for data to be manipulated as needed. Mongodb also has a restriction around storing dates only as UTC.

All timezones are stored by their IANA names so it's compatible with browser APIs.

We handle timezone calculations via a new package @date-fns/tz which we will be using in the future for handling timezone aware scheduled publishing/unpublishing and more.

UI

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Light mode

Left to do:

• Add translation for the Timezone: label
• Add exports for new utilities from payload and ui packages
• Add e2e tests for deeply nested fields inside tabs, arrays and blocks
• Add test for custom timezone options
• Add docs