feat: modal component

[fran-ink]

main usage is:

<Modal id="x">
  ({ closeModal }) => ... // modal content goes here
</Modal>

We can then open it with:

const { openModal } = useModalContext("x");

The ModalLayout namespace suggests layouts to use inside the modal (currently only one, CallToAction), but you can put whatever you want in the modal

[ink-victor]

LGTM