Document LIGHT_SHADOW_COLOR for canvas item shader

(cherry picked from commit 8baa1c7c7af2505724e5535acbbfd7031d8a58fa)

reference/shading_language.rst

@@ -447,6 +447,8 @@ CanvasItem (2D) - LightShader
 +-------------------------------------+-------------------------------------------------------------------------------+
 | const color *LIGHT\_COLOR*          | Color of Light                                                                |
 +-------------------------------------+-------------------------------------------------------------------------------+
+| const color *LIGHT\_SHADOW\_COLOR*  | Color of Light shadow                                                         |
++-------------------------------------+-------------------------------------------------------------------------------+
 | vec2 *LIGHT\_UV*                    | UV for light image                                                            |
 +-------------------------------------+-------------------------------------------------------------------------------+
 | color *SHADOW*                      | Light shadow color override                                                   |