RFC: add assertion utility to test if a value is "callable"

## 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 adding an assertion utility to test whether a value is callable. A value is callable if the value is a function, generator, or an ES6 class.

Prior art:

- [is-callable](https://github.com/ljharb/is-callable/blob/6fb3fbe1e63e87bb32b00ed34b67632edc6062b6/index.js)

Package: `@stdlib/assert/is-callable`
Alias: `isCallable`

## Related Issues

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

No.

## Questions

> Any questions for reviewers?

No.

## 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/develop/CODE_OF_CONDUCT.md

[faq]: https://github.com/stdlib-js/stdlib/blob/develop/FAQ.md