git: Add `hunk_style` setting (#26038)

This PR adds the `git.hunk_style` setting, allowing setting an alternate
style for hunks – specifically the rendering of unstaged hunks.

It has 2 options:

- `transparent` (unstaged hunks are more transparent/less opaque than
staged hunks)
- `pattern (unstaged hunks are indicated by a visual pattern)

We'll possibly explore a VSCode-style "don't show staged hunks", but the
complexity it adds is a bit out of scope for now.

Transparent:

![CleanShot 2025-03-04 at 09 07
09@2x](https://github.com/user-attachments/assets/a74c4286-8264-48a2-bd58-0c582efb4e22)

Pattern:

![CleanShot 2025-03-04 at 09 10
12@2x](https://github.com/user-attachments/assets/4dd3040e-fb36-4670-9279-fcc7a4f12ced)

Release Notes:

- Git Beta: Added `git.hunk_style` setting to allow toggling between git
hunk visual styles.