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

scripts/install-integrations.js

@@ -0,0 +1,27 @@
+let fs = require('fs/promises')
+let { spawn } = require('child_process')
+let path = require('path')
+let root = process.cwd()
+
+function npmInstall(cwd) {
+  return new Promise((resolve) => {
+    let childProcess = spawn('npm', ['install'], { cwd })
+    childProcess.on('exit', resolve)
+  })
+}
+
+async function install() {
+  let base = path.resolve(root, 'integrations')
+  let ignoreFolders = ['node_modules']
+  let integrations = (await fs.readdir(base, { withFileTypes: true }))
+    .filter((integration) => integration.isDirectory())
+    .filter((integration) => !ignoreFolders.includes(integration.name))
+    .map((folder) => path.resolve(base, folder.name))
+    .concat([base])
+    .map((integration) => npmInstall(integration))
+
+  await Promise.all(integrations)
+  console.log('Done!')
+}
+
+install()