Add linting commands for JS, CSS, and PHP (#175)

* Add linting commands for JS, CSS, and PHP

Setup is copied from wc-admin, except for `install-if-deps-outdated`.

Commands added:
- npm run lint # for both JS and CSS
- npm run lint:css
- npm run lint:css-fix # lint and fix CSS
- npm run lint:js
- npm run lint:php
- npm run lint:php-fix # lint and fix PHP

.eslintignore

@@ -0,0 +1,2 @@
+bin/*
+!bin/generate-docs

.eslintrc

@@ -0,0 +1,37 @@
+/** @format */
+
+{
+	"root": true,
+	"parser": "babel-eslint",
+	"extends": [ "wpcalypso/react", "plugin:jsx-a11y/recommended" ],
+	"plugins": [ "jsx-a11y", "jest" ],
+	"env": {
+		"browser": true,
+		"jest/globals": true,
+		"node": true,
+	},
+	"globals": {
+		"wp": true,
+		"wpApiSettings": true,
+		"wcSettings": true,
+	},
+	"settings": {
+		"react": {
+			"version": "detect",
+		},
+	},
+	"rules": {
+		"camelcase": 0,
+		"indent": 0,
+		"max-len": [ 2, { "code": 140 } ],
+		"no-console": 1,
+		"react/no-danger": 0,
+		"react/react-in-jsx-scope": 0,
+		"wpcalypso/import-no-redux-combine-reducers": 0,
+		"wpcalypso/jsx-classname-namespace": 0,
+		"wpcalypso/redux-no-bound-selectors": 1,
+		"jsx-a11y/label-has-for": [ "error", {
+			"required": "id"
+		} ]
+	},
+}

.stylelintrc.json

@@ -0,0 +1,20 @@
+{
+	"extends": "stylelint-config-wordpress",
+	"rules": {
+		"at-rule-empty-line-before": null,
+		"at-rule-no-unknown": null,
+		"comment-empty-line-before": null,
+		"declaration-block-no-duplicate-properties": null,
+		"declaration-colon-newline-after": null,
+		"declaration-property-unit-whitelist": null,
+		"font-weight-notation": null,
+		"max-line-length": null,
+		"no-descending-specificity": null,
+		"no-duplicate-selectors": null,
+		"rule-empty-line-before": null,
+		"selector-class-pattern": null,
+		"string-quotes": "single",
+		"value-keyword-case": null,
+		"value-list-comma-newline-after": null
+	}
+}