No entry field for `icon-padding`

**Maputnik version**: v2.1.1 - [web page](https://maplibre.org/maputnik)
**Browser**: Firefox 128.3.1esr, Chrome 130.0.6723.70
**OS**: Win 11 Pro

**Description of the bug**:
There is no entry field for `icon-padding` property of `symbol` layers.

**Steps to reproduce the behavior**:
1. Create a new symbol layer or select a symbol layer of an existing style
2. Expand "Icon layout properties", if needed
3. Attempt to enter a value for `padding`

**Style file or style URL**:
N/A

**Screenshots**:

Start in the symbol layer editor, under the “Icon layout properties” controls, and compare the available inputs with the `icon-padding` property in the MapLibre Style Specification. Add an entry field for `icon-padding` where users can enter a value, then verify it appears when creating or editing a symbol layer.

Written by the indexing model from the issue text.