Button Groups do not support File Buttons

[endigo9740]

Current Behavior

Users on Discord have pointed out that Button Groups do not currently support the use of a embedded File Button component.

Steps To Reproduce

The reason for this is Button Groups expect the child button elements to be native <button> elements without styles applied:
https://www.skeleton.dev/elements/buttons#button-groups

<div class="btn-group-vertical variant-filled">
	<button>Months</button>
	<button>Days</button>
	<button>Years</button>
</div>

However, the File Button has a hardcoded .btn class implemented, meaning it has default styles which breaks this use case:
https://github.com/skeletonlabs/skeleton/blob/master/packages/skeleton/src/lib/components/FileButton/FileButton.svelte#L22

More Information

We should move to make the .btn style a customizable prop. Perhaps included in the existing button prop styles along with the variant definition. As there may be other scenarios where an unstyled button is preferred.