Support customizing internal icons

[techniq]

It would be nice to remove direct usage of @mdi/js and allow the user to specify the icon library (still use material icons, or font awesome, feather, etc). Using something like unplugin-icons might be the key to open up this capability. We would need to add a mapping for each icon used within Svelte UX (things like next/previous arrows, etc). Initially would need to solve #142 first.

I've been thinking about this on/off. Creating this issue mostly to not forget about it.