Integrations setup (#4354)

* add integration test tools

* setup jest in the integrations folder

* add `test:integrations` script

The default `npm test` script will ignore all the tests in the
`integrations` folder.

* add integration tests with `webpack 5`

* add integration tests with `postcss-cli`

* add `npm run install:integrations` script

This script will run `npm install` in every integration, and in the
integrations folder itself (to setup Jest for example).

* add `toIncludeCss` custom matcher

* increate Jest timeout for integration tests

* add integration tests with `vite`

* add integration tests with `webpack 4`

* add isomorphic fetch

* add the ability to wait for specific stdout/stderr output

* write vite tests, assert using API calls

We will wait for the correct stdout/stderr output, once we know that we
can request the fresh css, we will fetch it and make assertions
accordingly.

Port is currently hardcoded, maybe we should use a packaage to ensure
that we use a free port.

* add integration tests with `rollup`

* add integration tests with `parcel`

* run all integration tests in band

* add .gitignore in integrations folder

integrations/postcss-cli/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "postcss-cli",
+  "private": true,
+  "version": "0.0.0",
+  "scripts": {
+    "build": "NODE_ENV=production postcss ./src/index.css -o ./dist/main.css",
+    "test": "jest"
+  },
+  "jest": {
+    "displayName": "PostCSS CLI",
+    "setupFilesAfterEnv": [
+      "<rootDir>/../../jest/customMatchers.js"
+    ]
+  },
+  "devDependencies": {
+    "postcss": "^8.2.15",
+    "postcss-cli": "^8.3.1"
+  }
+}