Merge OSC1337-instance with SIXEL-instance

[junjihashimoto]

To support hterm of chromeos and iterm2, I've added OSC1337.
But ToOSC-typesclass is separate from ToSixel, so the usability is low.

[junjihashimoto]

I am thinking using environment variables to detect terminal condition.
TERM variable of crostini is xterm-256color, though it does not support sixel.
So we have to define custom environment variable to choose OSC1337 or SIXEL.
How about using TERM_GRAPHICS variable?

[AutumnMeowMeow]

Unfortunately iTerm2 never made a clear means of identifying its image feature as a separate thing, unlike sixel support which is reported in device attributes. These days the XTVERSION sequence (see xterm ctlseqs) is used to detect terminals with iTerm2 (which AFAIK are hterm, iTerm2, mintty, and wezterm).