deploy
Copy mappingsFile into <geyserPluginDir>/custom_mappings/, creating the directory if needed. If geyserPluginDir is null (proxy admin has no Geyser plugin installed), this is a no-op; the mappings stay in the proxy's work/merged/ folder for manual copy.
Parameters
geyserPluginDir
typically the result of detectGeyserPluginDir; may be null when no Geyser plugin is present.
mappingsFile
the merged Geyser mappings JSON to copy.
logger
where to send the one-line "deployed to ..." message.