Add eslint to enforce no-prototype-builtins

The way to prevent issues like #58 is not obvious, and I would expect
not all programmers to have memorised the contents of
`Object.prototype`, so I think it would be prudent to check for such
issues automatically.

The no-prototype-builtins rule is enforced automatically by the set of
rules enabled in `.eslintrc.json` by

    "extends": "eslint:recommended"

It's a great sign that all the other enabled rules also pass without
modifications!

I've added the eslint invocation to `package.json`'s `posttest` field.
This should make the check minimally intrusive while developing, as it
will only run when the functional tests (via `jest`) pass first.

.eslintrc.json

@@ -0,0 +1,15 @@
+{
+    "env": {
+        "browser": true,
+        "es2021": true,
+        "node": true,
+        "jest": true
+    },
+    "extends": "eslint:recommended",
+    "parserOptions": {
+        "ecmaVersion": 12,
+        "sourceType": "module"
+    },
+    "rules": {
+    }
+}

package.json

@@ -15,7 +15,8 @@
     "test": "jest",
     "test:coverage": "yarn test -- --coverage",
     "test:report": "cat ./coverage/lcov.info | ./node_modules/.bin/coveralls",
-    "test:watch": "yarn test -- --watch"
+    "test:watch": "yarn test -- --watch",
+    "posttest": "eslint src/"
   },
   "author": "Matt Phillips",
   "license": "MIT",
@@ -30,6 +31,7 @@
     "babel-preset-es2015": "^6.18.0",
     "babel-preset-stage-0": "^6.16.0",
     "coveralls": "^3.0.0",
+    "eslint": "^7.10.0",
     "jest": "^23.6.0"
   },
   "babel": {