Add ArrayFlat type

[Emiyaaaaa]

• closed Proposal: Flat (equivalent to Array.prototype.flat()) #1075

[Copilot]

Pull Request Overview

This pull request adds tests for the new ArrayFlat type and updates the README to document its behavior.

• Added comprehensive tests in test-d/array-flat.ts to verify ArrayFlat across various array structures and depth parameters.
• Updated readme.md to include an entry for ArrayFlat, describing its behavior similar to Array#flat().

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test-d/array-flat.ts	Added tests for the ArrayFlat type covering basic, nested, explicit depth, union, rest elements, and readonly arrays cases.
readme.md	Added documentation for the ArrayFlat type.

Comments suppressed due to low confidence (3)

test-d/array-flat.ts:13

• [nitpick] Consider renaming 'Arr' to a more descriptive name like 'NestedArray' to improve clarity.

type Arr = [[0, [1, [2, [3, [4, [5]]]]]]];

test-d/array-flat.ts:40

• [nitpick] Consider clarifying the intent behind using 'Array<3>'. If it is meant to represent an array of the literal 3, a more descriptive type alias could improve readability.

expectType<ArrayFlat<[1, [2, ...Array<3>], 4]>>([1, 2, ...(null! as Array<3>), 4]);

readme.md:288

• [nitpick] Consider expanding the description to clarify how 'ArrayFlat' handles explicit depths and union types, which may help users better understand its usage.

+ [`ArrayFlat`](source/array-flat.d.ts) - Creates a new array type by flattening an array to a specified depth, just like `Array#flat()`.

[som-sm]

@Emiyaaaaa Added comments on the tests — haven’t reviewed the implementation yet.