fix(utils): add isString

Author: koory1st

packages/utils/src/index.js

@@ -1,3 +1,4 @@
 export { isNull } from './is_null.js';
 export { isNumber } from './is_number.js';
 export { isUndefined } from './is_undefined.js';
+export { isString } from './is_string.js';

packages/utils/src/is_string.js

@@ -0,0 +1,3 @@
+export function isString(v) {
+  return typeof v === 'string';
+}