[ic-side-navigation/ic-navigation-item] when selected and focused the icon is not centred

## Summary of the bug
When you focus a selected navigation item in the collapsed state, the icon is not in the centre of the focus.

Need design input on this because if we add a margin so that the icon is centred within the focus then it won't align with the rest of the navigation items which are unfocused.

## 🪜 How to reproduce
Tell us the steps to reproduce the problem:
1. Go to any side nav example
2. Focus the selected navigation item
3. See the error

## 📸 Screenshots or code

## 🧐 Expected behaviour
The icon should be centred within the focus.

Start with the ic-side-navigation/ic-navigation-item component and reproduce the issue in any side nav example by focusing the selected item in its collapsed state. Compare the focused icon position with the focus area and the other navigation items. Done means the icon is centred within the focus while the navigation remains visually consistent.

Written by the indexing model from the issue text.