[Feature Request]: option to customize pinned buffer icon 📌

### What?

It would be great to have an option to customize the pin icon.
Or maybe to remove it and use colors to indicate pinned state.
It is hardcoded now:
```
builtin.pinned = Group:new({
id = PINNED_ID,
name = PINNED_NAME,
icon = "📌",
priority = 1,
separator = {
style = separator.none,
},
})
```

Alternatively, I’m looking for a simple way to change it dynamically.

### Why?

_No response_