[RFC]: add commands for easy switching and previewing of syntax highlighting themes in the REPL

### Description

This RFC proposes

- adding commands like `prevTheme` and `nextTheme` for easier switching between syntax highlighting themes
- adding a command like `themeSample` that prints out a syntactically highlighted sample code to demonstrate how the current theme looks like.

### Related Issues

Related discussions #2341

### Questions

No.

### Other

No.

### 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:`.

Start by reviewing the REPL command-handling entry points and the syntax-highlighting theme implementation, then read related discussion #2341. Done means the REPL supports commands for cycling themes and displaying a highlighted sample of the current theme, with behavior agreed through the RFC discussion.

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