Add rootContext to webpack types (#29242)

types/webpack/index.d.ts

@@ -1578,6 +1578,11 @@ declare namespace webpack {
              */
             context: string;
 
+            /**
+             * Starting with webpack 4, the formerly `this.options.context` is provided as `this.rootContext`.
+             */
+            rootContext: string;
+
             /**
              * The resolved request string.
              * In the example: "/abc/loader1.js?xyz!/abc/node_modules/loader2/index.js!/abc/resource.js?rrr"