Add type support for const/readonly arrays in .each cases

[IgnusG]

Clear and concise description of the problem

The current type specification does not allow const or readonly arrays so the following works but does not include proper types inside test:

test.each([
    [1, ["1"]],
    [2, ["1", "2"]],
    [3, ["1", "2", "3"]],
  ])("renders pagination for a total of %i pages", (total, expectedElements) => {
    // total is typed as number | string[]
    // expectedElements is typed as number | string[]
  });

The quick-fix marking the input array as readonly does not typecheck:

// however each does not accept readonly tuples
test.each([
    [1, ["1"]],
    [2, ["1", "2"]],
    [3, ["1", "2", "3"]],
  ] as const)("renders pagination for a total of %i pages", (total, expectedElements) => {
    // total would be typed as 1 | 2 | 3
    // expectedElements is typed as ["1"] | ["1", "2"] ...
  });

Suggested solution

The following change to the EachFunction type will do the trick:

declare type EachFunction = <T>(cases: T[] | readonly T[]) => (name: string, fn: (...args: T extends any[] | readonly any[] ? { -readonly [k in keyof T]: T[k] } : [T]) => void) => void;

Alternative

No response

Additional context

No response

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[sheremet-va]

PR is welcome 🙏