Core/Movement: Replace old TargetedMovementGenerator into ChaseMovementGenerator and FollowMovementGenerator, full rewrite for both.

- Chase to angle is now functional. Pets use this to chase behind the target. Closes #19925.
- Chase to arbitrary range interval works. Not used anywhere, but you can technically make hunter-like mobs.
- Pets now follow the hunter cleanly and without stutter stepping. Also fix some other things. Closes #8924.

(cherry picked from commit 2a84562dc8)

src/server/game/Movement/AbstractFollower.cpp

@@ -0,0 +1,31 @@
+/*
+ * This file is part of the TrinityCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+ 
+#include "AbstractFollower.h"
+#include "Unit.h"
+
+void AbstractFollower::SetTarget(Unit* unit)
+{
+    if (unit == _target)
+        return;
+
+    if (_target)
+        _target->FollowerRemoved(this);
+    _target = unit;
+    if (_target)
+        _target->FollowerAdded(this);
+}