[RFC]: add `@stdlib/assert/is-palindrome`

### Description

This RFC proposes to add a new assert util to check if a string is palindrome or not.

### Related Issues

No.

### Questions

No.

### Other

Reference: [Palindrome | Wikipedia](https://en.wikipedia.org/wiki/Palindrome)

### 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:`.

The issue names no implementation file, test, or entry point. Start by locating existing @stdlib/assert utilities and their tests, then compare their conventions and confirm the palindrome behavior with maintainers before deciding what files define done.

索引モデルが issue の本文から書いたものです。