refactor: update BarBottom to display any icon

the component uses slots

icons availables:

https://icones.js.org/

slidev docs:

https://sli.dev/guide/syntax.html#icons

example:

<BarBottom  title="Slidev theme purplin">
  <Item text="slidevjs/slidev">
    <carbon:logo-github />
  </Item>
  <Item text="Slidevjs">
    <carbon:logo-twitter />
  </Item>
  <Item text="sli.dev">
    <carbon:link />
  </Item>
</BarBottom>

moudev / slidev-theme-purplin

refactor: update BarBottom to display any icon #4