fix(gen-rollup-conf): don't create CSS maps for standalone (#158)

The CSS is injected into JS with the sourcemaps. This bloats the files
without much of a benefit (they would be very rarly used, most people
won't use them at all).

Saves about 50 KiB (~10%) from minified Vis Timeline.

Sourcemaps are still generated for CSS files as they're external with
very little added to the CSS file itself.

Author: Thomaash

src/module/generate-rollup-configuration/index.ts

@@ -265,7 +265,8 @@ const generateRollupPluginArray = (
       extract: !injectCSS && `styles/${fullLibraryFilename}.css`,
       inject: injectCSS,
       minimize,
-      sourceMap: true,
+      // Prevent the sourcemaps from being injected into JS files.
+      sourceMap: !injectCSS,
       plugins: [
         postcssAssetsPlugin({
           loadPaths: [assets]