Add devdev typescript.

Author: paulmillr

package.json

@@ -52,7 +52,8 @@
     "eslint": "^7.0.0",
     "mocha": "^7.1.1",
     "nyc": "^15.0.0",
-    "rimraf": "^3.0.0"
+    "rimraf": "^3.0.0",
+    "typescript": "^4.0.3"
   },
   "nyc": {
     "reporter": [