Optionally show magic link on sign-up page

[mhkeller]

What kind of change does this PR introduce?

Feature

What is the current behavior?

When you sign-up, it gives the user the option of signing in with a magic link.

What is the new behavior?

You can set a boolean to not show a magic link if you don't want to support that login option.

Additional context

Closes https://github.com/supabase-community/supabase-ui-svelte/issues/4