Fixes #6818

This is a convenience feature that exists in other editors, allowing
quick creation a new tab by double-clicking the empty space on the tab
bar:
![2024-02-26 14 26
16](https://github.com/zed-industries/zed/assets/601206/55d99a84-2e61-494d-b06c-6e5f15071655)

Release Notes:

- Added the ability to open a new buffer when double-clicking on the tab
bar ([#6818](https://github.com/zed-industries/zed/issues/6818)).