[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:`.

该 issue 没有指定实现文件、测试或入口点。先定位现有的 @stdlib/assert 工具及其测试，然后比较它们的约定，并在决定哪些文件定义了完成标准之前，与维护者确认回文行为。

由索引模型根据 Issue 内容生成。