Add `rust-toolchain.toml` config file

[zooeywm]

Please describe the problem you're trying to solve

The yazi terminal file manager is the best one I've ever seen. But the first time when I try to contribute, I have to manually switch channel to nightly.

And there is one deprecated fmt option, which recommended by using show_parse_errors instead of hide_parse_errors

Would you be willing to contribute this feature?

• [X] Yes, I'll give it a shot

Describe the solution you'd like

Add a rust-toolchain.toml in the project's root directory to automatically switch channel.

Additional context

No response