Add ESLint configuration files

.eslintignore

+locale
+data
+preferences
+webextension

.eslintrc

+{
+  "env": {
+    "es6": true
+  },
+  "globals": {
+    "exports": true,
+    "module": true,
+    "require": true,
+    "Services": true
+  },
+  "extends": [
+    "eslint:recommended",
+    "plugin:no-unsanitized/DOM"
+  ],
+  "overrides": {
+    "files": [
+      "test/*"
+    ],
+    "rules": {
+      "brace-style": "off",
+      "function-paren-newline": "off",
+      "max-len": "off"
+    }
+  },
+  "plugins": [
+    "no-unsanitized"
+  ],
+  "rules": {
+    "array-bracket-newline": "error",
+    "array-bracket-spacing": "error",
+    "arrow-body-style": "error",
+    "arrow-parens": "error",
+    "arrow-spacing": "error",
+    "block-spacing": "error",
+    "brace-style": "error",
+    "camelcase": "error",
+    "comma-spacing": "error",
+    "comma-style": "error",
+    "computed-property-spacing": "error",
+    "consistent-this": "error",
+    "curly": "error",
+    "eol-last": "error",
+    "eqeqeq": "error",
+    "func-call-spacing": "error",
+    "function-paren-newline": "error",
+    "generator-star-spacing": "error",
+    "indent": [
+      "error",
+      4
+    ],
+    "key-spacing": "error",
+    "keyword-spacing": "error",
+    "linebreak-style": [
+      "error",
+      "unix"
+    ],
+    "new-parens": "error",
+    "no-array-constructor": "error",
+    "no-bitwise": "error",
+    "no-confusing-arrow": "error",
+    "no-continue": "error",
+    "no-duplicate-imports": "error",
+    "no-eval": "error",
+    "no-extend-native": "error",
+    "no-implicit-coercion": "error",
+    "no-implied-eval": "error",
+    "no-invalid-this": "error",
+    "no-iterator": "error",
+    "no-label-var": "error",
+    "no-labels": "error",
+    "no-lone-blocks": "error",
+    "no-loop-func": "error",
+    "no-multi-spaces": "error",
+    "no-multi-str": "error",
+    "no-multiple-empty-lines": [
+      "error", {
+        "max": 1,
+        "maxEOF": 1,
+        "maxBOF": 0
+      }
+    ],
+    "max-len": [
+      "error", {
+        "code": 120
+      }
+    ],
+    "no-negated-condition": "error",
+    "no-new": "error",
+    "no-new-func": "error",
+    "no-new-object": "error",
+    "no-new-wrappers": "error",
+    "no-octal-escape": "error",
+    "no-proto": "error",
+    "no-return-assign": "error",
+    "no-return-await": "error",
+    "no-script-url": "error",
+    "no-self-compare": "error",
+    "no-sequences": "error",
+    "no-shadow-restricted-names": "error",
+    "no-tabs": "error",
+    "no-ternary": "error",
+    "no-throw-literal": "error",
+    "no-trailing-spaces": "error",
+    "no-undef-init": "error",
+    "no-unmodified-loop-condition": "error",
+    "no-unused-expressions": "error",
+    "no-useless-call": "error",
+    "no-useless-concat": "error",
+    "no-useless-constructor": "error",
+    "no-useless-rename": "error",
+    "no-useless-return": "error",
+    "no-void": "error",
+    "no-warning-comments": "warn",
+    "no-whitespace-before-property": "error",
+    "no-with": "error",
+    "object-curly-spacing": "error",
+    "object-shorthand": [
+      "error",
+      "consistent-as-needed"
+    ],
+    "operator-assignment": "error",
+    "operator-linebreak": "error",
+    "prefer-numeric-literals": "error",
+    "prefer-promise-reject-errors": "error",
+    "quote-props": "error",
+    "prefer-rest-params": "error",
+    "prefer-spread": "error",
+    "prefer-template": "error",
+    "quotes": [
+      "error",
+      "single"
+    ],
+    "rest-spread-spacing": "error",
+    "require-await": "error",
+    "semi": "error",
+    "semi-spacing": "error",
+    "semi-style": "error",
+    "space-before-blocks": "error",
+    "space-before-function-paren": "error",
+    "space-in-parens": "error",
+    "space-infix-ops": "error",
+    "space-unary-ops": "error",
+    "strict": [
+      "error",
+      "global"
+    ],
+    "switch-colon-spacing": "error",
+    "symbol-description": "error",
+    "template-curly-spacing": "error",
+    "template-tag-spacing": "error",
+    "unicode-bom": "error",
+    "vars-on-top": "error",
+    "wrap-regex": "error",
+    "yield-star-spacing": "error",
+    "yoda": "error"
+  }
+}