Kitty graphics: support feature queries (e key) to detect Unicode placeholder support

^{Originally posted by ayn2op October 25, 2026} Kitty added a way for clients to ask which parts of the graphics protocol a terminal supports (kitty#10732, spec). It would be great if Ghostty supported it too.

Problem: a client can't tell whether a terminal supports Unicode placeholders (U=1). Konsole answers OK to a=q but ignores U=1, so it prints the placeholder characters as text. Clients like mine (discordo) end up hardcoding a list of terminal names instead.

How it works: a query with e= makes the terminal include in its reply every key from the query that it supports for that action, plus e itself:

query: <ESC>_Ga=q,i=31,e=T,U=1<ESC>\ reply: <ESC>_Gi=31,e=T,U=1;OK<ESC>\

If a key is missing from the reply, it isn't supported. A feature query transmits no image data and has no side effects.

Ghostty already supports Unicode placeholders, so for that case it only needs to echo the keys back. Kitty's implementation is in commit 4bafcb9.