convert utils/ to typescript (reduxjs#3534)

Former-commit-id: 01968f6
Former-commit-id: b3d2147

Author: cellog

src/utils/actionTypes.js renamed to src/utils/actionTypes.ts

@@ -2,7 +2,7 @@
  * @param {any} obj The object to inspect.
  * @returns {boolean} True if the argument appears to be a plain object.
  */
-export default function isPlainObject(obj) {
+export default function isPlainObject(obj: unknown) {
   if (typeof obj !== 'object' || obj === null) return false
 
   let proto = obj

src/utils/isPlainObject.js renamed to src/utils/isPlainObject.ts

@@ -4,7 +4,7 @@
  * @param {String} message The warning message.
  * @returns {void}
  */
-export default function warning(message) {
+export default function warning(message: string) {
   /* eslint-disable no-console */
   if (typeof console !== 'undefined' && typeof console.error === 'function') {
     console.error(message)