Add 'currently sleeping' status indicator

## Suggestion

A per-world status indicator on the action bar and/or boss bar that shows how many people are sleeping

Suggested on spigot by https://www.spigotmc.org/members/michelle1492.1771172/

## Objectives

- [ ] Per-world action bar and/or boss bar status indicator
- [ ] Auto-disable bar locations if the MC server version doesn't implement them
- [ ] Configurable message in `settings.yml`
- [ ] Translation of legacy color codes from the config string
- [ ] Placeholders for world name, player sleeping count, online player count, percentage sleeping

## Concerns

- Vanished players support
- We don't want to expose the *actual* player count, only the amount of online players that the player can see (`Player#canSee` iirc). But also handle cases e.g. where 1 remaining person who isn't sleeping is vanished. Vanish plugins should be tagging them as sleepingIgnored anyway.
- Feature creep.