Introduce the concept of Contacts.Chunk

This commit introduces a new class `Contacts.Chunk`. Just like libfolks,
we see a contact as a collection of data, or to word it differently: a
collection built up from "chunks" of information.

The net result of adding this concept adds quite a bit of lines of code,
but it does have some major benefits:

* Rather than stuffing new properties into yet another if-else spread
  out over multiple places in contacts-utils (and quite a bit of other
  files), we can create a new subclass of `Contacts.Chunk`
* This also goes for property-specific logic, which we can consolidate
  within their appropriate classes/files.
* All of our logic is now unit-testable

In the future, this would allow for more cleanups/features:

* We can put the serialization code for each property inside the
  `Contacts.Chunk`
* We can extend ContactSheet to show a vCard's information, before
  actually importing it into a Folks.Individual.
* We can write unit tests on the set of chunks, rather than regularly
  having to deal with yet another regression in e.g. the birthday
  editor.

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