unify partial deep type & improve `defaults` function examples

[mesqueeb]

• unify PartialDeep to use

export type PartialDeep<O> = O extends Record<string | number | symbol, unknown>
  ? { [K in keyof O]?: PartialDeep<O[K]> }
  : O

• export this type as well
• make sure that merge-anything works correctly with the PartialDeep type I end up using

[DOCS]

• rewrite the documentation for the defaults... functions to use PartialDeep
  • suggest to use merge() function for cases with nested objects
• explain how to be re-use modules with config in other modules with other where filters