Add OptionalPaths type

[som-sm]

Closes #1426

[sindresorhus]

the new type says it can be used with PickDeep/OmitDeep, but that does not hold for all of the documented output. For example:

type Tuple = [string, number?];
type Result = OmitDeep<Tuple, OptionalPaths<Tuple>>;

OptionalPaths<Tuple> includes 1 | '1', while the deep path helpers only accept the string tuple path here. So the numeric member makes the composition fail. I think the simplest fix is to make the public result only produce paths that are accepted by the existing deep helpers, instead of returning extra numeric tuple variants.

There is also a generic version of the same problem, which is close to the original use case from the issue:

type MinimalObject<T extends object> = OmitDeep<T, OptionalPaths<T>>;

That currently fails the path constraint. Extract<OptionalPaths<T>, Paths<T>> makes the object case work, but users should not need that if the documented use case is direct composition with OmitDeep.

and I would add tests for:

type RequiredUserFields = OmitDeep<UserFields, OptionalPaths<UserFields>>;
type OptionalUserFields = PickDeep<UserFields, OptionalPaths<UserFields>>;
type MinimalObject<T extends object> = OmitDeep<T, OptionalPaths<T>>;

And if array support stays in scope, also add a tuple composition test. Right now the standalone OptionalPaths tests are pretty thorough, but they miss whether the type actually compsoes with the helpers it is meant to feed into.