fix: replace setImmediate with setTimeout

[sidwebworks]

Currently, the batchInsert function uses setImmediate to schedule the recursive insert calls to avoid blocking the event loop.

However, this causes the API to break in the browser where setImmediate is not defined as it's a Node API.

This PR fixes that issue by using setTimeout with a delay of 0.

cc// @micheleriva

[micheleriva]

Thanks! Lgtm