DECCKM (`ESC [?1h`/`ESC [?1l`) controls whether cursor keys send
application sequences (e.g. `ESC OD`) or standard ANSI cursor sequences
(`ESC [D`).
However, xterm only uses this alternative sequence when no modifiers
are held. This can be tested by running `echo -e '\e[?1h'; cat` and then
pressing some of the cursor keys:
The cursor left key prints `^[OD` (as expected), but Shift+Left prints
`^[[1;2D`, same for the other cursor keys and other modifiers (Ctrl and
Alt).
To match this xterm behavior, only use the application sequence when
no modifiers are held.
This makes the Ctrl+{Left,Right} keybinds work in neovim.
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