Allow piping data into the CLI (#6876)

* use outputFile instead of direct writeFile

This is an improvement we introduced earlier but forgot this part.

* allow to pipe in data to the CLI

* add integration tests to validate piping to the CLI

* update changelog

integrations/tailwindcss-cli/tests/integration.test.js

@@ -27,6 +27,23 @@ describe('static build', () => {
     )
   })
 
+  it('should be possible to pipe in data', async () => {
+    await writeInputFile('index.html', html`<div class="font-bold"></div>`)
+
+    await $('cat ./src/index.css | node ../../lib/cli.js -i - -o ./dist/main.css', {
+      shell: true,
+      env: { NODE_ENV: 'production' },
+    })
+
+    expect(await readOutputFile('main.css')).toIncludeCss(
+      css`
+        .font-bold {
+          font-weight: 700;
+        }
+      `
+    )
+  })
+
   it('should safelist a list of classes to always include', async () => {
     await writeInputFile('index.html', html`<div class="font-bold"></div>`)
     await writeInputFile(