This PR adds settings for the Git Panel.

The new settings include:

| Setting | Description | Default |
|---------|-------------|---------|
| `git_panel.button` | Toggle visibility of the Git Panel button in the
status bar | `true` |
| `git_panel.dock` | Choose where to dock the Git Panel | `"left"` |
| `git_panel.default_width` | Set the default width of the Git Panel in
pixels | `360` |
| `git_panel.status_style` | Select how Git status is displayed |
`"icon"` |
| `git_panel.scrollbar.show` | Configure scrollbar behavior | Inherits
from editor settings |

Example usage:

```json
"git_panel": {
  "button": true,
  "dock": "left",
  "default_width": 360,
  "status_style": "icon",
  "scrollbar": {
    "show": "auto"
  }
}
```

Release Notes:

- N/A