Add @s and @p selector support (#6110)

Adds support for @s and @p player selectors (which both select the
source player).

Closes #5925
Partially Addresses #6006

Author: JRoy

Essentials/src/main/java/com/earth2me/essentials/Essentials.java

@@ -957,6 +957,10 @@ public User matchUser(final Server server, final User sourceUser, final String s
         final User user;
         Player exPlayer;
 
+        if (sourceUser != null && (searchTerm.equals("@p") || searchTerm.equals("@s"))) {
+            return sourceUser;
+        }
+
         try {
             exPlayer = server.getPlayer(UUID.fromString(searchTerm));
         } catch (final IllegalArgumentException ex) {

Essentials/src/main/java/com/earth2me/essentials/commands/EssentialsLoopCommand.java

@@ -28,6 +28,11 @@ protected void loopOfflinePlayersConsumer(final Server server, final CommandSour
             throw new PlayerNotFoundException();
         }
 
+        if (sender.isPlayer() && (searchTerm.equals("@s") || searchTerm.equals("@p"))) {
+            userConsumer.accept((User) sender.getUser());
+            return;
+        }
+
         final UUID uuid = StringUtil.toUUID(searchTerm);
         if (uuid != null) {
             final User matchedUser = ess.getUser(uuid);
@@ -78,6 +83,11 @@ protected void loopOnlinePlayersConsumer(final Server server, final CommandSourc
             throw new PlayerNotFoundException();
         }
 
+        if (sender.isPlayer() && (searchTerm.equals("@s") || searchTerm.equals("@p"))) {
+            userConsumer.accept((User) sender.getUser());
+            return;
+        }
+
         final boolean skipHidden = sender.isPlayer() && !ess.getUser(sender.getPlayer()).canInteractVanished();
 
         if (matchWildcards && (searchTerm.contentEquals("**") || searchTerm.contentEquals("*"))) {