[RFC]: Add `@stdlib/string/for-each-right`

### Description

This RFC proposes adding support for invoking a callback for each (visual) character of a string, while iterating from right-to-left.

Signature:

```javascript
forEachRight( str, clbk[, thisArg] )
```

where `clbk` (similar to [`@stdlib/utils/for-each-right`](https://github.com/stdlib-js/stdlib/tree/ee2d245f666240fc94f6bdfcf78d62f1883773a8/lib/node_modules/%40stdlib/utils/for-each-right)) is provided

- **value**: visual character
- **idx**: starting character index
- **str**: input string

Package: `@stdlib/string/for-each-right`
Alias: `forEachRight`

### Related Issues

None.

### Questions

No.

### Other

Will require iterating over grapheme cluster breaks. See [`@stdlib/string/reverse`](https://github.com/stdlib-js/stdlib/blob/develop/lib/node_modules/%40stdlib/string/reverse) for such an example.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.