docs: fix preset layout (#13377)

- add newline after preset json
- declare preset json as json

tools/docs/presets.ts

@@ -48,10 +48,10 @@ export async function generatePresets(dist: string): Promise<void> {
       } else {
         logger.warn(`Preset ${name}:${preset} has no description`);
       }
-      body += '\n```\n';
+      body += '\n```json\n';
       body += JSON.stringify(value, null, 2);
       body += '\n```\n';
-      body += '----\n';
+      body += '\n----\n';
       if (body.includes('{{arg0}}')) {
         header += '(`<arg0>`';
         if (body.includes('{{arg1}}')) {