Format/slightly modernize the JS (#345)

* add basic JS tooling

* fix accidental uses of global variables

* auto-format

* add and fix a couple more standard lint rules

* remove useless return false from settimeout callbacks

* document JS contributing

* fix whitespace in package.json

* add JS stuff to codespell skiplist

* codespell take two

* update github action and add comments about duplicated logic

.eslintrc.js

@@ -0,0 +1,17 @@
+'use strict';
+module.exports = {
+  extends: ['eslint:recommended', 'eslint-config-prettier'],
+  parserOptions: {
+    ecmaVersion: 2020,
+  },
+  env: {
+    browser: true,
+  },
+  rules: {
+    strict: ['error', 'global'],
+    'no-unused-vars': ['error', { vars: 'local' }],
+    eqeqeq: ['error', 'always', { null: 'ignore' }],
+    curly: ['error', 'multi-line'],
+    'no-var': 'error',
+  },
+};