Update and properly configure eslint

.eslintrc

@@ -1,3 +1,21 @@
 {
-  "extends": "eslint:recommended"
+  "extends": "eslint:recommended",
+  "parserOptions": {
+    "ecmaVersion": 11,
+    "sourceType": "module"
+  },
+  "env": {
+    "es6": true,
+    "node": true
+  },
+  "overrides": [
+    {
+      "files": [
+        "tests/**/*.js"
+      ],
+      "env": {
+        "jest": true
+      }
+    }
+  ]  
 }

package.json

@@ -33,7 +33,7 @@
     "browserify": "^16.5.0",
     "canvas": "^2.6.1",
     "codemirror": "~5.49.2",
-    "eslint": "^6.6.0",
+    "eslint": "^7.8.1",
     "iconv-lite": "^0.5.0",
     "jest": "^26.4.2",
     "jest-screenshot": "^0.3.1",
@@ -59,6 +59,7 @@
     "pdf-guide": "node docs/generate.js",
     "website": "node docs/generate_website.js",
     "docs": "npm run pdf-guide && npm run website && npm run browser-demo",
+    "lint": "eslint {lib,tests}/**/*.js",
     "prettier": "prettier {lib,tests,demo,docs}/**/*.js",
     "test": "jest -i",
     "test:visual": "jest visual/ -i",