test: add coverage report

Author: haoqunjiang

Diff for: .gitignore

@@ -5,3 +5,4 @@ dist
 link.sh
 .cache
 TODOs.md
+coverage

Diff for: package.json

@@ -12,7 +12,7 @@
     "dev": "tsc --watch",
     "build": "tsc",
     "pretest": "tsc",
-    "test": "jest",
+    "test": "jest --coverage",
     "dev-example": "webpack-dev-server --config example/webpack.config.js --inline --hot",
     "build-example": "rm -rf example/dist && webpack --config example/webpack.config.js --env.prod",
     "lint": "prettier --write --parser typescript \"{src,test}/**/*.{j,t}s\"",