Highlight whitespace-only inline completions with background (#21954)

Noticed that whitespace-only insertions are really hard to make out, so
this changes it to make them visible by giving them a green background.

![screenshot-2024-12-13-10 49
09@2x](https://github.com/user-attachments/assets/10d83067-46f2-4cb5-97fa-0f44d254890d)


Release Notes:

- N/A

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Co-authored-by: Antonio <antonio@zed.dev>