detectGeyserPluginDir
Look for any Geyser-* subdirectory under proxyPluginsDir. Geyser ships separate jars for each proxy platform (Geyser-Velocity, Geyser-BungeeCord) — they all create a same-named plugin folder. First match wins; in practice there's only ever one Geyser plugin loaded per proxy.
Return
the matched plugin directory, or null if none found.
Parameters
preferredDirectoryName
exact (case-insensitive) folder name to try first; may be null or blank to skip the preference.