chore: stringify sync payload before putting it in the console

This allow to have them displayed by playwright in the python
console.

Co-authored-by: David Larlet <david@larlet.fr>

umap/static/umap/js/modules/sync/engine.js

@@ -450,5 +450,5 @@ export class Operations {
 }
 
 function debug(...args) {
-  console.debug('SYNC ⇆', ...args)
+  console.debug('SYNC ⇆', ...args.map((x) => JSON.stringify(x)))
 }