first attempt at moving modal to using meltui and adding a popover

[jamesscottbrown]

What does this change?

This switches to using meltui instead of @rgossiaux/svelte-headlessui to implement the modals, and adds a popover component.

Why?

How?

Related issues:

Does this introduce new dependencies?

How is it tested?

How is it documented?

Are light and dark themes considered?

Is it complete?

• [ ] Have you included changeset file?
• [ ] If this adds a new component, is it exported via index.js?

[jamesscottbrown]

This has been superceeded by #240