feat: SetEnv variable

[harsxv]

I encountered an issue while using my new terminal, GhostTTY:

[[email protected] ~]# clear
'xterm-ghostty': unknown terminal type.
[[email protected] ~]# echo $?
1

The fix was to add SetEnv in the SSH config. Turns out that there was a feature request as well here: #371

So I made this commit and PR to address.

Tested it, and LGTM. Please review it, thanks!

[[email protected] ~]# clear
[[email protected] ~]# echo $?
0

[ErebusBat]

@moul would it be possible to get this merged?

Would you be willing to add some maintainers to the repository?