Button: add classname as props

[Thykof]

this would allow me to do:

<Button className="w-full" variant="secondary" preIcon={<FiPlus />}>
  {Intl.t('account.add')}
</Button>

because w-full as been removed from the Button component, I need to add it in some places.

[mazmassa]

Humm, not sure about that! This is tricky because we don't know in the API signature what className we are overriding. 🤔

There's many others className in the component implementation.

[Thykof]

we do this #147 instead