replaced single commands with /pixelblocks root command and permissions

New /pixelblocks (alias /pb) with create, give, exit and destroyall
subcommands, each gated by a permission (exit defaults to true, the
rest to op). Fixes the args[0] crash and missing UUID validation in
give, and the offline-owner NPE plus concurrent-modification risk in
destroyall (destroy now takes a force flag that skips the ownership
check for admins). Also centralizes the item id tag in
PixelBlockItem.setBlockId and drops the overridden itemName leftover.

src/main/resources/plugin.yml

@@ -3,7 +3,20 @@ version: '${version}'
 main: eu.mhsl.minecraft.pixelblocks.Main
 api-version: '1.21'
 commands:
-  createpixelblock:
-  exitworld:
-  givepixelblock:
-  destroypixelblocks:
+  pixelblocks:
+    description: Verwaltung von Pixelblöcken
+    usage: /pixelblocks <create|give|exit|destroyall>
+    aliases: [pb]
+permissions:
+  pixelblocks.command.create:
+    description: Erlaubt das Erstellen eines Pixelblocks per Befehl
+    default: op
+  pixelblocks.command.give:
+    description: Erlaubt das Geben von Pixelblock-Items
+    default: op
+  pixelblocks.command.exit:
+    description: Erlaubt das Verlassen eines Pixelblocks per Befehl
+    default: true
+  pixelblocks.command.destroyall:
+    description: Erlaubt das Zerstören aller Pixelblöcke
+    default: op