feat: add variant: list to schema.multiselect

[jeremydw]

• create an alternative to mantine's MultiSelecft
• supports inline editing, deletion, reordering, etc.
• appears like a minimal version of the normal array field
• add test to verify they serialize/deserialize the same way

usage:

schema.multiselect({
  variant: 'list' // default is 'multiselect';
});

[stevenle]

i think this change is fine, although i think i may want to play around with the ux a little bit. i think i'd prefer the bordered text inputs over the unbordered ones so that it's clear that the user can edit the field.

idiomatically i think we should consider making this a new field type, e.g. schema.stringlist() or schema.strings() since in my mind the term "multiselect" assumes a ui that includes a select list. users in the future would wonder why we chose to do schema.multiselect({variant: 'list'}) instead of schema.list({variant: 'multiselect'}) and so i'd like to avoid that type of head scratching.

[stevenle]

another thing to consider is that with multiselect there's an assumption of uniqueness, whereas from your example, the string list could accept ["foo", "bar", "foo"]. let's create a new field for this, happy to accept suggestions for what to call it.