#549: Add Fluid types / tags

By: Martoph <sager1018@gmail.com>

paper-api/src/main/java/org/bukkit/Fluid.java

@@ -0,0 +1,24 @@
+package org.bukkit;
+
+import java.util.Locale;
+import org.jetbrains.annotations.NotNull;
+
+public enum Fluid implements Keyed {
+
+    WATER,
+    FLOWING_WATER,
+    LAVA,
+    FLOWING_LAVA;
+
+    private final NamespacedKey key;
+
+    private Fluid() {
+        this.key = NamespacedKey.minecraft(this.name().toLowerCase(Locale.ROOT));
+    }
+
+    @NotNull
+    @Override
+    public NamespacedKey getKey() {
+        return key;
+    }
+}