RFC: add deep-set-by

## Checklist

> Please ensure the following tasks are completed before filing an issue.

* [x] Read and understood the [Code of Conduct][code-of-conduct].
* [x] Searched for existing issues and pull requests.
* [x] If this is a general question, searched the [FAQ][faq] for an existing answer.
* [x] If this is a feature request, the issue name begins with `RFC: `.

## Description

> Description of the issue (or feature request).

This RFC proposes to add `@stdlib/utils/deep-set-by`. Currently, when `@stdlib/utils/deep-set` is provided a `function` as a value, the `deepSet` implementation treats the value as a callback. This prevents being able to set a deeply nested value in a straightforward manner.

To overcome this limitation, this proposal suggests moving the callback implementation to a separate package (`@stdlib/utils/deep-set-by`) and changing `@stdlib/utils/deep-set` to drop support for a callback argument.

## Related Issues

> Does this issue (or feature request) have any related issues?

No.

## Questions

> Any questions for reviewers?

One question is whether the name `deep-set-by` is appropriate. The names matches the suffix for `omit-by` and `pick-by`; however, the implementation details differ for the proposed package compared to the existing convention. Notably, `omit-by` and `pick-by` deal with predicates. In this case, a provided function is a setter, rather than a predicate.

## Other

> Any other information relevant to this issue (or feature request)? This may include screenshots, references, stack traces, sample output, and/or implementation notes.

No.

[code-of-conduct]: https://github.com/stdlib-js/stdlib/blob/master/CODE_OF_CONDUCT.md
[faq]: https://github.com/stdlib-js/stdlib/blob/master/FAQ.md