Allow configuring the window zoom increment/step

It would be useful to have a setting to control the increment used by the **Zoom In** / **Zoom Out** commands (`Ctrl++` / `Ctrl+-`).

Currently, each step changes the zoom by 20%, which can feel too large, especially when working across monitors with different sizes or resolutions. While `window.zoomLevel` supports fractional values, these finer adjustments cannot be conveniently made with the keyboard shortcuts.

For example, a setting such as:

```json
"window.zoomStep": 0.25
```

could allow users to choose a finer zoom increment while keeping the existing value as the default.