Add new type: DeepMutable<T>

[dosentmatter]

Is your feature request related to a real problem or use-case?

Related to #27 and #111.
We have DeepPartial<T>, DeepRequired<T>, and DeepReadonly<T>. We should also add DeepMutable<T>.

Similar to #112, maybe we should have a DeepOptional<T> that would be an alias of DeepPartial<T>.

Describe a solution including usage in code example

export type DeepMutable<T> = T extends ((...args: any[]) => any) | Primitive | null | undefined
  ? T
  : T extends _DeepMutableArray<any>
  ? _DeepMutableArray<T[number]>
: T extends _DeepMutableObject<infer U>
  ? _DeepMutableObject<U>
  : never;
/** @private */
// tslint:disable-next-line:class-name
export interface _DeepMutableArray<T> extends Array<DeepMutable<T>> {}
/** @private */
export type _DeepMutableObject<T> = {
  -readonly [P in keyof T]: DeepMutable<T[P]>
};

Who does this impact? Who is this for?

TypeScript users.

[piotrwitek]

Hey, thanks for the suggestion, it would be great to have this type.
Contributions are welcome!