[META] Add timezone support to core date fields

Problem/Motivation

✨ [PP-1] Add ability to select a timezone for datetime field Postponed currently has 117 followers and we're on comment #290. The latest patch is 98k. It risks never being completed. The scope is too huge, there are too many moving pieces. It needs to be split up and tackled in smaller, more actionable pieces.

Proposed resolution

@jonathanshaw proposed a way to split it up into an actionable plan at #2632040-107: [PP-1] Add ability to select a timezone for datetime field → , and other users (@dww, @WidgetsBurritos, @amjad1233, et al) support the approach.

This issue is a parent meta plan to coordinate the moving pieces. Once this is finally complete, we can reply at the parent and mark it fixed, notifying all the followers when the feature is working and committed.

High-level overview of the plan:

• Refactor datetime formatters to simplify the logic.
• Give settings to the datetime widgets (shared in common with the formatters, and date_range) that allow sitebuilders to control the time zone used to interpret input.
• Add a new (optional) property to the date field (and date_range) that stores the preferred time zone.
• Create thorough tests to verify that all this arcane time zone handling is working correctly everywhere.

Remaining tasks

Child issues

1. 📌 Create TimezoneFieldTest class Needs work
2. #3185729: Refactor datetime formatters →
3. #3185730: Move formatter timezone settings into a trait →
4. #3185733: Apply timezone trait to datetime widgets →
5. #3185734: Add per-instance datetime timezone storage →

General points (needs triage)

• Add tests for the widget and formatter settings forms
• Extend tests to cover date range
• Extend tests for the full range of formatters and widgets
• Debug the date_range formatter
• Various minor cleanups, debugging and testing per #105.
• Upgrade path tests
• Finish/reroll the datetime element once #2799987: Datetime and Datelist element's timezone handling is fragile, buggy and wrongly documented → gets fixed
• Add constraint test coverage if 🐛 Add a format and start/end validation constraints to DateRangeItem to provide helpful REST/JSON:API error message Needs work gets resolved before this issue does
• Address deprecated tests per #242.1
• Resolve issue with JSON:API's handling of datetime data structure as described in #242.2
• Add migration-related test coverage per #245.1

User interface changes

A timezone handling configuration drop down on field storage configuration, and if configured for per-date storage, a timezone selector on the date widget. See child issues for details.

Screenshots

API changes

• The datetime form element will have an optional #expose_timezone boolean, that if TRUE, will add a timezone selector to the element.
• Date time widgets and formatters will have various new methods. Possibly getDefaultValue() should be public.

See child issues for details.

Data model changes

Datetime storage will store a timezone per date.

See child issues.

Release notes snippet

@todo