[@svelteui/core]: Use `default` variant in button when variant is not specified

[1yne]

Description

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The current button uses the filled variant when the variant prop is not given. As pointed by @piersg in #357, it should instead use the default variant

[BeeMargarida]

I'm gonna leave here what I wrote in the issue, since this change has more ramifications than it looks. I'm gonna close since this is not the best way of solving this, but thank you for your work!

I understand your point, but by default we want to filled variant for Button, like we want hover variant for ActionIcon. Perhaps the change here is renaming the default variant instead of changing the defaults