Add support for grouped/list translations in translation stores

[MerryDev]

Adventure's translation stores currently map a translation key to exactly one translation value. This works well for regular messages,
but makes multi-line messages somewhat inconvenient when translations are stored in .properties files.
For example, a help message may currently be represented as separate translation keys:

command.help.0=<gray>---------------------
command.help.1=<yellow>/help <gray>- Shows this help
command.help.2=<yellow>/reload <gray>- Reloads the plugin
command.help.3=<gray>---------------------

There currently does not appear to be an API for retrieving these related translations as a logical group/list. This is particularly noticeable when using MiniMessageTranslationStore, where lists of translated MiniMessage components are a fairly natural use case. It could be useful to provide an API for retrieving grouped translation.

Something like this would be great to have

List<Component> group = store.group(key);

--> e.g. group.forEach(player::sendMessage);

[TheMolkaPL]

I'm not sure I fully understand the use case here.

What exactly is there to translate in command.help.0?
What is the semantic distinction between command.help.0 and command.help.3?

As far as I can tell, both are just independent translation strings. From the translation store's perspective, I don't see what makes them a logical group rather than two unrelated keys.