Mark cli as perma-unstable (using instability)

[MabezDev]

Just like we do in esp-hal

[bugadani]

Please elaborate while you still remember what you mean.

[jessebraham]

I'm not sure I understand. The cli feature is not even enabled when building documentation for docs.rs, what exactly will this accomplish?

I also do not have a high opinion of instability to begin with and would prefer to avoid it, it's been quite frustrating to work with.

[MabezDev]

Ah I missed this when I was building locally: https://github.com/esp-rs/espflash/blob/main/espflash/Cargo.toml#L19