Add '/paper mobcaps' and '/paper playermobcaps'

Add commands to get the mobcaps for a world, as well as the mobcaps for
each player when per-player mob spawning is enabled.

Also has a hover text on each mob category listing what entity types are
in said category

paper-server/src/test/java/io/papermc/paper/command/subcommands/MobcapsCommandTest.java

@@ -0,0 +1,22 @@
+package io.papermc.paper.command.subcommands;
+
+import java.util.HashSet;
+import java.util.Set;
+import net.minecraft.world.entity.MobCategory;
+import org.bukkit.support.environment.Normal;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+@Normal
+public class MobcapsCommandTest {
+    @Test
+    public void testMobCategoryColors() {
+        final Set<String> missing = new HashSet<>();
+        for (final MobCategory value : MobCategory.values()) {
+            if (!MobcapsCommand.MOB_CATEGORY_COLORS.containsKey(value)) {
+                missing.add(value.getName());
+            }
+        }
+        Assertions.assertTrue(missing.isEmpty(), "MobcapsCommand.MOB_CATEGORY_COLORS map missing TextColors for [" + String.join(", ", missing + "]"));
+    }
+}