Add placeholder files for 1.21

v1_21_R1/build.gradle.kts

@@ -0,0 +1,3 @@
+dependencies {
+    paperweight.paperDevBundle("1.21-R0.1-SNAPSHOT")
+}

v1_21_R1/src/main/java/dev/plex/itemizerx.v1_21_R1/Main.java

@@ -0,0 +1,5 @@
+package dev.plex.itemizerx.v1_21_R1;
+
+public class Main
+{
+}