Detect URLs at beginning and end of buffer as well (#9212)

Release Notes:

- Fixed URL detection at very beginning and very end of buffer
([#9210](https://github.com/zed-industries/zed/issues/9210)).

| Before | After |
| ------ | ----- |
| ![Screenshot 2024-03-12 at 07 09
52](https://github.com/zed-industries/zed/assets/38924/5c0beb28-f63b-4455-a5cd-31cec2d9a775)
| ![Screenshot 2024-03-12 at 07 11
03](https://github.com/zed-industries/zed/assets/38924/116f76a2-21e8-4c77-ac6e-a4e3c594b09d)
|