fix(sixel): accept tmux/screen TERM when tmux forwards Sixel (#14)

`kitty-gfx--sixel-detect' currently gates on the outer TERM matching
`xterm|vt[0-9]|foot|contour'.  Once `tmux-ok' has confirmed tmux >=
3.4, tmux itself owns the Sixel rendering -- the DCS payload never
reaches the outer terminal.  At that point the outer TERM regex is
irrelevant, but the canonical tmux assignment `tmux-256color' (and
the legacy `screen-256color') still trip the check, and detection
returns nil even when Sixel works fine.

Accept `tmux*' and `screen*' TERMs when `tmux-ok' is true.  Outside
tmux the existing regex still rules, so no behavioural change for
non-tmux setups.

Repro before this patch:

    TERM=tmux-256color emacs -nw -Q -l kitty-graphics.el \
        --eval '(setq kitty-gfx-debug t)' \
        --eval '(kitty-graphics-mode 1)'
    => "Kitty graphics: terminal not supported"
    *kitty-gfx-debug*: sixel-detect: nil (TERM=tmux-256color ... tmux-ok=yes)

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