RFC: evaluate a Legendre polynomial at a value `x`

## Checklist

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- [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).

Add support for evaluating a [Legendre polynomial](https://en.wikipedia.org/wiki/Legendre_polynomials) at a value `x`.

Package: `@stdlib/math/base/special/legendre-polynomial`
Alias: `base.legendrepoly`

Dependent on an implementation for the Gauss 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 \[[1](https://docs.scipy.org/doc/scipy/reference/generated/scipy.special.eval_legendre.html#scipy.special.eval_legendre)\]\[[2](https://github.com/scipy/scipy/blob/fe5fd89f1d13791f8bee620e424fe8f91fbecb02/scipy/special/orthogonal_eval.pxd#L365)\]
- [Wolfram](http://reference.wolfram.com/language/ref/LegendreP.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