Better support for property groups#1

Two approaches 1. Have an extra `groups?: Record<string, PartialVCard>;` property on vCards, which includes the grouped properties. Then, user code could decide whether it wants - Items ungrouped: Just use the pre-existing structure, ignoring `group` field in properties, or - Items grouped: Read through `groups` and then ignore all values which have a `group` field 2. Have a `groupVCard()` function, which groups all entries by group name (and those without, into `ungrouped`) into a `Record<string, PartialVCard>` I am currently favoring 2, as groups are rarely used and most consumers will not want to bother about it. This also reduces code size for group-agnostic consumers.