RequireAtLeastOne: Make other given keys optional

[ulken]

For each mapped type with given required key, make all other given keys optional.

Fixes #59

[sindresorhus]

From the description:

The remaining keys are kept as is.

That's not really true anymore, right?

[ulken]

That's not really true anymore, right?

I get the confusion, but I believe it still holds. It gets a little hairy with the terminology, but I'll try to explain the differences, as I understand it:

• Remaining keys: keys not given as parameters, i.e. keys of ObjectType not present in KeysType. Note: if no keys are given, then KeysType includes all keys of ObjectType.
• Other given keys: all keys of KeysType except the one we're currently mapping over.

Example

type SystemMessages = {
  default: string;
  
  macos?: string;
  linux?: string;
  windows?: string;

  optional?: string;
}

type ValidMessages = RequireAtLeastOne<
  SystemMessages,
  "macos" | "linux" | "windows"
>

In this case, default and optional are remaining keys and they are left intact/as is.

When we're creating the type where "macos" is required, "linux" and "windows" are the other given keys.

Does that make sense?

The change this PR introduces is making the other given keys optional.

[ulken]

@sindresorhus do you feel me?

[sindresorhus]

That makes sense. Thanks for explaining.