lib: restrict SASS plugin to scss files

To aid our effort to move away from SASS to pure CSS explicitly restrict
the esbuild SASS plugin to only take SASS files. Every Cockpit project
can now slowly convert SASS files to CSS where possible.

pkg/lib/esbuild-common.js

@@ -20,6 +20,7 @@ export const esbuildStylesPlugins = [
     }),
     sassPlugin({
         loadPaths: [...nodePaths, 'node_modules'],
+        filter: /\.scss/,
         quietDeps: true,
         async transform(source, resolveDir, path) {
             if (path.includes('patternfly-5-cockpit.scss')) {