Assign permission to the command (#3599)

Assigning permission allows people to possibly deny that permission to hide the command from tab complete list.
Permission is defaulted to true, so command keeps current behavior by default.

bootstrap/spigot/src/main/resources/plugin.yml

@@ -8,4 +8,8 @@ api-version: 1.13
 commands:
   geyser:
     description: The main command for Geyser.
-    usage: /geyser <subcommand>
+    usage: /geyser <subcommand>
+    permission: geyser.command
+permissions:
+  geyser.command:
+    default: true