fix(CSS): sourcemap support for PostCSS files (#754)

* fix(CSS): sourcemap support

* Create hip-spies-happen.md

Author: stipsan

.changeset/hip-spies-happen.md

@@ -0,0 +1,5 @@
+---
+'microbundle': patch
+---
+
+Enable sourcemaps for CSS

src/index.js

@@ -457,6 +457,7 @@ function createConfig(options, entry, format, writeMeta) {
 						// only write out CSS for the first bundle (avoids pointless extra files):
 						inject: false,
 						extract: !!writeMeta,
+						sourceMap: options.sourcemap,
 					}),
 					moduleAliases.length > 0 &&
 						alias({

test/__snapshots__/index.test.js.snap

@@ -433,6 +433,7 @@ Directory tree:
 basic-css
   dist
     basic-css.css
+    basic-css.css.map
     basic-css.esm.js
     basic-css.esm.js.map
     basic-css.js
@@ -454,9 +455,12 @@ Build \\"basicCss\\" to dist:
 134 B: basic-css.umd.js.br"
 `;
 
-exports[`fixtures build basic-css with microbundle 2`] = `7`;
+exports[`fixtures build basic-css with microbundle 2`] = `8`;
 
-exports[`fixtures build basic-css with microbundle 3`] = `"._rHGVB{display:flex;color:red;background:#00f}"`;
+exports[`fixtures build basic-css with microbundle 3`] = `
+"._rHGVB{display:flex;color:red;background:#00f}
+/*# sourceMappingURL=basic-css.css.map */"
+`;
 
 exports[`fixtures build basic-css with microbundle 4`] = `
 "export default function(){var e=document.createElement(\\"div\\");return e.className=\\"testing\\",e}