[6.x] Create an "auto" theme for the code editor field to support light and dark modes

[aaronbushnell]

Bug description

Currently, the code editor field supports either "Light" or "Dark" modes, but not "Auto". However, with Statamic supporting dark mode it would be nice if there was a way for the field to also adapt to this change instead of forcing one versus the other.

How to reproduce

1. Create a code field
2. Use "Light" or "Dark" as the theme option
3. Toggle dark mode

Logs

Environment

Environment
Application Name: Statamic Starter
Laravel Version: 12.26.3
PHP Version: 8.4.12
Composer Version: 2.8.10
Environment: local
Debug Mode: ENABLED
URL: statamic-starter.test
Maintenance Mode: OFF
Timezone: America/Indiana/Indianapolis
Locale: en

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: sqlite
Logs: stack / single
Mail: log
Queue: sync
Session: file

Storage
public/storage: LINKED

Statamic
Addons: 3
Sites: 1
Stache Watcher: Enabled (auto)
Static Caching: Disabled
Version: 6.0.0-alpha.5 PRO

Statamic Addons
rias/statamic-redirect: dev-statamic-6

Installation

Other (please explain)

Additional details

Created using our project starter