Add support for kitty keyboard protocol

[vamolessa]

https://sw.kovidgoyal.net/kitty/keyboard-protocol/

[vamolessa]

Will need some help with this since I don't have access to a linux pc anymore.
I did lay down the ground work for this however.
Key is now a struct instead of an enum, which now can contain independent information of key modifiers (shift, ctrl and alt).
Now it's just a matter of correctly implementing the Terminal::parse_keys function located at pepper/src/platforms/unix_utils.rs.