chore(cli): split compilers

packages/vant-cli/src/compiler/compile-css.ts

@@ -0,0 +1,12 @@
+import postcss from 'postcss';
+import postcssrc from 'postcss-load-config';
+import { POSTCSS_CONFIG_FILE } from '../common/constant';
+
+export async function compileCss(source: string | Buffer) {
+  const config = await postcssrc({}, POSTCSS_CONFIG_FILE);
+  const output = await postcss(config.plugins as any).process(source, {
+    from: undefined
+  });
+
+  return output;
+}