feat(terminal): add support for kitty keyboard protocol

This commit adds basic support for the kitty keyboard protocol to
Neovim's builtin terminal. For now only the first mode ("Disambiguate
escape codes") is supported.

test/unit/vterm_spec.lua

@@ -2324,6 +2324,9 @@ putglyph 1f3f4,200d,2620,fe0f 2 0,4]])
     local vt = init()
     local state = wantstate(vt)
 
+    -- Disambiguate escape codes
+    push('\x1b[>1u', vt)
+
     -- Unmodified ASCII
     inchar(41, vt)
     expect('output 29')
@@ -2478,6 +2481,8 @@ putglyph 1f3f4,200d,2620,fe0f 2 0,4]])
     expect_output('\x1b[I')
     vterm.vterm_state_focus_out(state)
     expect_output('\x1b[O')
+
+    push('\x1b[<u', vt)
   end)
 
   itp('26state_query', function()