refactor: generate postcss.config.js instead of .postcssrc.js

Author: yyx990803

packages/@vue/cli/__tests__/Generator.spec.js

@@ -649,7 +649,7 @@ test('extract config files', async () => {
   const js = v => `module.exports = ${stringifyJS(v, null, 2)}`
   expect(fs.readFileSync('/vue.config.js', 'utf-8')).toMatch(js(configs.vue))
   expect(fs.readFileSync('/babel.config.js', 'utf-8')).toMatch(js(configs.babel))
-  expect(fs.readFileSync('/.postcssrc.js', 'utf-8')).toMatch(js(configs.postcss))
+  expect(fs.readFileSync('/postcss.config.js', 'utf-8')).toMatch(js(configs.postcss))
   expect(fs.readFileSync('/.eslintrc.js', 'utf-8')).toMatch(js(configs.eslintConfig))
   expect(fs.readFileSync('/jest.config.js', 'utf-8')).toMatch(js(configs.jest))
   expect(fs.readFileSync('/.browserslistrc', 'utf-8')).toMatch('> 1%\nnot <= IE8')

packages/@vue/cli/lib/Generator.js

@@ -26,7 +26,7 @@ const defaultConfigTransforms = {
   }),
   postcss: new ConfigTransform({
     file: {
-      js: ['.postcssrc.js'],
+      js: ['postcss.config.js'],
       json: ['.postcssrc.json', '.postcssrc'],
       yaml: ['.postcssrc.yaml', '.postcssrc.yml']
     }