[FR]: Styles for non-editable columns

## Use case

Allow styling columns that are not editable to be slightly darker so it's clear to the user.
## Proposed solution

A new class so you can do something like:
```css
.npc-rdg-grid .npc-rdg-column-readonly {
background-color: color-mix(in oklch, var(--muted) 78%, var(--foreground) 8%);
color: var(--muted-foreground);
}
```