Prefer isSingleton, not a singleton function

[Noviny]

This pull request removes the singleton function in favour of users configuring the list instead, with isSingleton.

We are opting to AVOID discretizing list into two discriminated types.
By introducing a singleton function, we may be confusing users into thinking that these are not actually lists.

Instead, users shouldn't have any confusion about whether a list with isSingleton: true is still simply a list.
When configured, the list simply has the following added behaviour traits that revolve around the list containing only 1 item (id: 1):

• Easy to use in the Admin UI by pathing directly to either create or the item with id: 1
• Easy to use in the GraphQL API, with query parameters that default to id: 1 for reduce verbosity when using singletons

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Name	Status	Preview	Updated
keystone-next-docs	✅ Ready (Inspect)	Visit Preview	Sep 20, 2022 at 4:44AM (UTC)

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[dcousens]

I don't know if this will be how we continue to do this by the next major, but I think this is the best compromise for now