Function flatten

flatten<A, D>(array, depth?): FlatArray<A, D>[]
Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Use Array.flat if possible or polyfill globally:
```
Array.prototype.flat = function(depth) {
    return flatten(this, depth)
}
```
Type Parameters
- A extends unknown[]
- D extends number = 1
Parameters
- array: A
- Optional depth: D
  The maximum recursion depth
Returns FlatArray<A, D>[]
Example
```
flatten([1, 2, [3, 4]])
// [1, 2, 3, 4]

flatten([1, 2, [3, 4, [5, 6]]])
// [1, 2, 3, 4, [5, 6]]

flatten([1, 2, [3, 4, [5, 6]]], 2)
// [1, 2, 3, 4, 5, 6]
```
Tutorial
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
- Defined in src/array.ts:272
flatten<A, D>(array, depth?): readonly FlatArray<A, D>[]
Type Parameters
- A extends readonly unknown[]
- D extends number = 1
Parameters
- array: A
- Optional depth: D
Returns readonly FlatArray<A, D>[]
- Defined in src/array.ts:276