Do less work when revealing entries in the outline panel (#20031)

Before this change, we were trying to determine current element before
debouncing, causing a lot of extra work on caret movement. Now, we only
do this for the task that managed to wait the entire debounce period.

Closes https://github.com/zed-industries/zed/issues/19817
Closes https://github.com/zed-industries/zed/issues/14235

Release Notes:

- Fixed outline panel-related performance issues when selections change
in the large document
([#19817](https://github.com/zed-industries/zed/issues/19817)),
([#14235](https://github.com/zed-industries/zed/issues/14235))