fix: recognize TERM=rxvt-unicode-256color

[dtolnay]

Fixes #1026.

I confirmed that cargo build --release && target/release/yazi and pressing q to exit no longer leaves characters on the next line.

target/release/yazi --debug >/dev/null still does, but I think that is intentional and fine. The --debug output contains:

...
Emulator
    Emulator.via_env: ("rxvt-unicode-256color", "")
    Emulator.via_csi: Ok(Unknown([]))
    Emulator.detect: Urxvt
...

According to https://www.arewesixelyet.com/#urxvt, SIXEL graphics format is not supported by this terminal emulator, but I wasn't sure how to determine whether Adaptor::Iterm2 would be supported.

[sxyazi]

but I wasn't sure how to determine whether Adaptor::Iterm2 would be supported

Inline images protocol does not specify how to detect whether the terminal supports it, so it can only rely on the builtin database of the app. I believe urxvt does not support it since I did not find any information about it.

[sxyazi]

LGTM, thanks for the fix!