Add an isValid() method to Entity. Addresses BUKKIT-810

By: TomyLobo <tomylobo@nurfuerspam.de>

paper-api/src/main/java/org/bukkit/entity/Entity.java

@@ -129,6 +129,13 @@ public interface Entity extends Metadatable {
      */
     public boolean isDead();
 
+    /**
+     * Returns false if the entity has died or been despawned for some other
+     * reason.
+     * @return True if valid.
+     */
+    public boolean isValid();
+
     /**
      * Gets the {@link Server} that contains this Entity
      *