return callback values in parser to allow synchronous parsing

lib/less/parser.js

@@ -472,10 +472,10 @@ less.Parser = function Parser(env) {
                         e = new(LessError)(e, env);
                     }
 
-                    callback(e);
+                    return callback(e);
                 }
                 else {
-                    callback(null, root);
+                    return callback(null, root);
                 }
             };
 
@@ -483,7 +483,7 @@ less.Parser = function Parser(env) {
                 new tree.importVisitor(this.imports, finish)
                     .run(root);
             } else {
-                finish();
+                return finish();
             }
         },