Updated Terser's mangle option

This'll keep the names of our classes and functions intact for neater stacktraces.
The [impact on file size](https://github.com/alphagov/govuk-frontend/issues/4076\#issuecomment-1677677788) is very acceptable in front of the maintainability gain.

rollup.release.config.mjs

@@ -30,7 +30,13 @@ export default defineConfig(({ i: input }) => ({
     plugins: [
       terser({
         format: { comments: false },
-        mangle: { reserved: Object.keys(GOVUKFrontend) },
+        mangle: {
+          keep_classnames: true,
+          keep_fnames: true,
+          // Ensure all top-level exports skip mangling, for example
+          // non-function string constants like `export { version }`
+          reserved: Object.keys(GOVUKFrontend)
+        },
 
         // Include sources content from source maps to inspect
         // GOV.UK Frontend and other dependencies' source code