Support more of the Kitty keyboard protocol

[willmcgugan]

In particular, support key release event so an app could detect long-presses.

[mgesbert]

We are currently working on unifying the experience across terminals and we face issue with various degree of support for the kitty protocol in Textual. Notably we are trying to bring shortcuts like alt+backspace into Warp and Ghostty.
Our top priority would be to improve Textual's support of the kitty keyboard protocol as per https://sw.kovidgoyal.net/kitty/keyboard-protocol/#progressive-enhancement.
In our case for alt+backspace to work it would require the support up to flag 8.