fix(auth-ui-svelte): make additionalData prop optional

[sergejcodes]

What kind of change does this PR introduce?

Bug fix

Previous behavior: TypeScript shows an error if the additionalData prop is not explicitly set on the Svelte Auth component.

Since additionalData is defined as optional in shared/src/types.ts it makes sense to also make it optional for the Svelte Auth component.

What is the new behavior?

The additionalData prop for the Svelte Auth component is now optional and doesn't need to be explicitly set. This prevents a scenario like this in TypeScript:

<script lang="ts">
  import { Auth } from '@supabase/auth-ui-svelte'
</script>

<Auth additionalData={undefined} />

[Blackwidow-sudo]

Good to see that there is already a PR.
I was following https://supabase.com/docs/guides/getting-started/tutorials/with-sveltekit#set-up-a-login-page and this error was annoying.

[silentworks]

Thank you for this PR.