tuple type spread

[KiaraGrouwstra]

This tuple type spread is part of my attempt at tackling #5453 following the tuple-flavored variant of the proposal laid out by @Artazor.

It adds a new spread node as a type-level equivalent of the one for the expression level. This could potentially be reused for another part of #5453 that'd require #6606, spreads in type-level function 'application' e.g. Fn(...Args).

Progress: evaluation with generics delayed too much.

type a = [1, ...[2]];
>a : [1, 2]
type Combine<Head, Tail extends any[]> = [Head, ...Tail];
>Combine : [Head, ...Tail]
type b = Combine<1, [2, 3]>;
>b : [1, ...Tail] 

[KiaraGrouwstra]

The challenge I'm facing here is re-evaluating the parent tuple after generics are known, so I could spread out the generic's tuple over the parent tuple. I've yet to really figure this out.

[typescript-bot]

Thanks for your contribution. This PR has not been updated in a while and cannot be automatically merged at the time being. For housekeeping purposes we are closing stale PRs. If you'd still like to continue working on this PR, please leave a message and one of the maintainers can reopen it.

[KiaraGrouwstra]

For posterity, #24897 (comment) offers an alternative we can use today.