Support the kitty keyboard protocol

[Qi Xiao]

What new feature should Elvish have?

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

Two things to figure out:

• Are the enabling and disabling sequences (CSI > 1 u and CSI < u) no-ops for terminals that don't support the protocol
• Is Kitty's encoding completely compatible existing encodings Elvish supports (either there's no overlap, or the overlapped sequences correspond to identical keys)

If both are true, this support can be enabled unconditionally; otherwise it will have to depend on whether the terminal supports the protocol

Output of "elvish -version"

0.21.0-dev.0.20240430145458-c7d81b20a010

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