#1029: Trial changing a small number of inner enums to classes/interfaces to better support custom values

This PR is a subset of the enum PR #660 and is designed as a low impact
trial run of the design and backwards compatibility to inform subsequent
development.

Additional plugin compatibility features may be available by setting
`settings.compatibility.enum-compatibility-mode` to `true` in
`bukkit.yml`.

By: DerFrZocker <derrieple@gmail.com>

paper-api/src/main/java/org/bukkit/util/OldEnum.java

@@ -0,0 +1,42 @@
+package org.bukkit.util;
+
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Class which holds common methods which are present in an enum.
+ *
+ * @param <T> the type of the old enum.
+ * @deprecated only for backwards compatibility.
+ */
+@ApiStatus.Internal
+@Deprecated(since = "1.21")
+public interface OldEnum<T extends OldEnum<T>> extends Comparable<T> {
+
+    /**
+     * @param other to compare to.
+     * @return negative if this old enum is lower, zero if equal and positive if
+     * higher than the given old enum.
+     * @deprecated only for backwards compatibility, old enums can not be
+     * compared.
+     */
+    @Deprecated(since = "1.21")
+    @Override
+    int compareTo(@NotNull T other);
+
+    /**
+     * @return the name of the old enum.
+     * @deprecated only for backwards compatibility.
+     */
+    @NotNull
+    @Deprecated(since = "1.21")
+    String name();
+
+    /**
+     * @return the ordinal of the old enum.
+     * @deprecated only for backwards compatibility, it is not guaranteed that
+     * an old enum always has the same ordinal.
+     */
+    @Deprecated(since = "1.21")
+    int ordinal();
+}