Update mask type to allow null in Group and Text

Changed the type of the 'mask' property in both Two.Group and Two.Text definitions to accept 'null' in addition to 'Shape' and 'undefined', improving type safety and clarity.

Author: jonobr1

types.d.ts

@@ -1724,7 +1724,7 @@ declare module 'two.js/src/group' {
      * @name Two.Group#mask
      * @property {Shape} - The Two.js object to clip from a group's rendering.
      */
-    mask: Shape | undefined;
+    mask: Shape | null | undefined;
     /**
      * @name Two.Group#strokeAttenuation
      * @property {Boolean} - When set to `true`, stroke width scales with transformations (default behavior). When `false`, stroke width remains constant in screen space for all child shapes.
@@ -4296,7 +4296,7 @@ declare module 'two.js/src/text' {
      * @property {Shape} - The shape whose alpha property becomes a clipping area for the text.
      * @nota-bene This property is currently not working becuase of SVG spec issues found here {@link https://code.google.com/p/chromium/issues/detail?id=370951}.
      */
-    mask: any;
+    mask: Shape | null | undefined;
     /**
      * @name Two.Text#clip
      * @property {Shape} - Object to define clipping area.