fix: upgrade typescript from 3.8.3 to 3.9.2 (#13)

Snyk has created this PR to upgrade typescript from 3.8.3 to 3.9.2.

See this package in NPM:
https://www.npmjs.com/package/typescript

See this project in Snyk:
https://app.snyk.io/org/wallace41290/project/948d3534-f195-42f8-b20c-055729fc9646?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package-lock.json

@@ -95,7 +95,7 @@
     "tslint-config-prettier": "^1.15.0",
     "tslint-config-standard": "^8.0.1",
     "typedoc": "^0.17.7",
-    "typescript": "^3.8.3"
+    "typescript": "^3.9.2"
   },
   "lint-staged": {
     "{src,test}/**/*.ts": [