Add ESLint (#3)

package.json

@@ -6,8 +6,9 @@
   "module": "my-element.js",
   "type": "module",
   "scripts": {
-    "build": "lit-analyzer && tsc",
+    "build": "tsc",
     "build:watch": "tsc --watch",
+    "lint": "lit-analyzer && eslint 'src/**/*.ts'",
     "serve": "es-dev-server --app-index demo/index.html --node-resolve --watch --open",
     "test": "karma start karma.conf.cjs",
     "test:watch": "karma start karma.conf.cjs --auto-watch=true --single-run=false",
@@ -29,9 +30,12 @@
     "@open-wc/testing-karma": "^3.2.19",
     "@types/chai": "^4.2.7",
     "@types/mocha": "^5.2.7",
+    "@typescript-eslint/eslint-plugin": "^2.12.0",
+    "@typescript-eslint/parser": "^2.12.0",
     "chai": "^4.2.0",
     "deepmerge": "^4.2.2",
     "es-dev-server": "^1.23.1",
+    "eslint": "^6.7.2",
     "karma": "^4.4.1",
     "karma-chai": "^0.1.0",
     "karma-mocha": "^1.3.0",