Add missing InputActionStateMonitorIndex file from last commit.

Author: Darren-Kelly-Unity

Packages/com.unity.inputsystem/InputSystem/Actions/InputActionStateMonitorIndex.cs

@@ -0,0 +1,42 @@
+namespace UnityEngine.InputSystem
+{
+    /// <summary>
+    /// Bit-packed id registered with <see cref="InputManager"/> for <see cref="InputActionState"/> state change monitors.
+    /// Layout must match <c>TriggerState.kMaxNum*</c> limits on <see cref="InputActionState"/>.
+    /// </summary>
+    internal readonly struct InputActionStateMonitorIndex
+    {
+        readonly long m_Packed;
+
+        public InputActionStateMonitorIndex(long packed)
+        {
+            m_Packed = packed;
+        }
+
+        public long Packed => m_Packed;
+
+        public static implicit operator long(InputActionStateMonitorIndex index) => index.m_Packed;
+
+        public static InputActionStateMonitorIndex FromPacked(long packed) => new InputActionStateMonitorIndex(packed);
+
+        public static InputActionStateMonitorIndex Create(int mapIndex, int controlIndex, int bindingIndex, int priority)
+        {
+            long result = controlIndex;
+            result |= (long)bindingIndex << 24;
+            result |= (long)mapIndex << 40;
+            result |= (long)priority << 48;
+            return new InputActionStateMonitorIndex(result);
+        }
+
+        public int ControlIndex => (int)(m_Packed & 0x00ffffff);
+
+        public int BindingIndex => (int)((m_Packed >> 24) & 0xffff);
+
+        public int MapIndex => (int)((m_Packed >> 40) & 0xff);
+
+        /// <summary>
+        /// Only the low 8 bits are stored; larger <see cref="InputAction.Priority"/> values truncate when packed.
+        /// </summary>
+        public int Priority => (int)((m_Packed >> 48) & 0xff);
+    }
+}