Field Default Values UI: Time zone options

https://git.drupalcode.org/project/xdate/-/blob/c4c9995862a16869c117ce21...

Problem/Motivation

Presently, the default time zone is proposed by plugins: weighted for order, able to be disabled, and obviously possible to add custom or additional ones. It could possibly be better to just use an event, but it does want a configuration (and UI) way of dis/enabling and weighting them.

There is presently only the Plugin configuration (and tests) for setting the default timezone.

Proposed resolution

Add a table drag of available plugins, with space to add configuration for them, and enable/disable them.