[RFC]: evaluate a Chebyshev polynomial of the second kind at a value `x`

## Checklist

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- [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).

Add support for evaluating a [Chebyshev polynomial](https://en.wikipedia.org/wiki/Chebyshev_polynomials) of the second kind at a value `x`.

Package: `@stdlib/math/base/special/chebyshev-u-polynomial`
Alias: `base.chebyshevupoly`

Implementation should be straightforward, but depends on having an implementation of the Guass hypergeometric function `hyp2f1`.

## 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.

Reference implementations:

- [SciPy](https://github.com/scipy/scipy/blob/2526df72e5d4ca8bad6e2f4b3cbdfbc33e805865/scipy/special/orthogonal_eval.pxd#L286)
- [SciPy](https://docs.scipy.org/doc/scipy/reference/generated/scipy.special.eval_chebyu.html#scipy.special.eval_chebyu)
- [Wolfram](http://reference.wolfram.com/language/ref/ChebyshevU.html)

[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