feat: add lang.yml messages for give/take commands and per-currency prefixes (#40)

* fix: give/take commands not sending currency change messages

Removes the previous vault-messages/balance-messages notify system
from #38, which never fired for /ecobits give/take/givesilent/takesilent
even when enabled — that's exactly how reward plugins (crates,
DeluxeMenus, ChatGames) award currency, so players never saw feedback.

Replaces it with dedicated received-currency/lost-currency lang.yml
keys sent directly to the affected player from give/take and their
silent variants, and fixes took-currency, which was referenced by
CommandTake but never defined.

Also adds a withCurrencyPlaceholders() helper to collapse the repeated
%amount%/%amount_short%/%amount_formatted%/.../%symbol% replace chains
duplicated across the currency commands.

* fix: keep givesilent/takesilent fully silent

Silent variants shouldn't message the recipient either — only give/take
(non-silent) send the received-currency/lost-currency feedback now.

* feat: per-currency chat prefixes

Adds an optional prefix: field to each currency in config.yml, used
instead of the plugin's global lang.yml prefix for that currency's
messages (give/take/pay/set/reset/balance/get). Falls back to the
global prefix when blank/unset.

Fixes the %currency% workaround some users tried in lang.yml's global
prefix: it never gets replaced there (the built-in prefix has no
currency context) and /ecobits reload showed it literally.

* refactor: inline messagePrefix into getCurrencyMessage

Only used in one place, per review feedback.