Move from headless-ui to melt-ui

[MrGreenTea]

We've used the svelte-headlessui package for some components. This was before we found melt-ui.

There are multiple reasons to remove svelte-headlessui:

• svelte-headlessui is not updated anymore
• svelte-headlessui causes some warnings
• melt-ui is more idiomatic for svelte

[MrGreenTea]

Components that still use headless-ui:

• https://github.com/YoungVision-eV/website/blob/029b6f5abef7844c9b17760054cfcdcb9fa3405f/src/components/support-us/Disclosure.svelte#L3
• https://github.com/YoungVision-eV/website/blob/029b6f5abef7844c9b17760054cfcdcb9fa3405f/src/components/support-us/DonationMethodTabs.svelte#L3