RFC: add `@stdlib/stats/base/dists/generalized-hyperbolic/mean`

## Description

This RFC proposes implementing the **mean** for the **Generalized Hyperbolic** distribution as package `@stdlib/stats/base/dists/generalized-hyperbolic/mean`.

## Related Issues

Parent: #5486

## Questions

No.

## Other

Implementation notes:

- Follow existing stdlib patterns for implementations of the various statistical distributions. See the [existing distribution implementations](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/@stdlib/stats/base/dists) for guidance.

## 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:`.