fix: setupPreparser should be synchronous

Author: kermanx

Diff for: packages/slidev/node/setups/preparser.ts

@@ -5,7 +5,7 @@ import { resolveAddons } from '../integrations/addons'
 import { getRoots } from '../resolver'
 import { loadSetups } from './load'
 
-export default async function setupPreparser() {
+export default function setupPreparser() {
   injectPreparserExtensionLoader(async (headmatter: Record<string, unknown>, filepath: string, mode?: string) => {
     const addons = headmatter?.addons as string[]
     if (!addons?.length)