Display a link for Rust Playground whenever AI replies with rust code

[joulei]

Whenever AI replies with a rust code block, we want to add a link/button to the Rust Playground website attaching the given code. (Later on to be expanded to support other languages/websites).

Requirements to display the button:

1. Message must be markdown
   In chat_line.rs the following heuristic is used to determine if the message is markdown (perhaps this can be reused):

        let to_markdown = parse_markdown(&text);
        let is_plain_text = to_markdown.nodes.len() <= 3;

2. Message must include a rust code block ```rs ... ``` or ```rust ... ```

Opening the playground

We should open the Rust Playground URL by using robius-open, e.g. robius_open::Uri::new(&some_url).open(). This is already being used in Moxin.

The Rust Playground website allows attaching code to its URL: https://play.rust-lang.org/?code=<your-encoded-code-here>.

For example, opening a Rust Playground including a simple hello world, can be done with the following URL:
https://play.rust-lang.org/?code=fn%20main()%20%7B%0A%20%20%20%20println!%28%22Hello%2C%20world!%22%29%3B%0A%7D

Design is up for discussion, but I would suggest using something like a small ferris (rust mascot) PNG. It can be gray, and turn orange (the ferris color) on hover.

[Guocork]

#254