tty-features: grant imagescroll to kitty by default

Verified today: a full-width pane in real Kitty (no pane-scrollbars, no
split, no other panes) correctly preserves a placed image through
sustained native scrolling (scroll-to-bottom, copy mode up/down,
mouse-wheel scrolling) - 21 scroll events produced only 3 full
redraws, all correlating with mode transitions rather than the scrolls
themselves, with zero fallback to the manual redraw path throughout.

margins is deliberately not added - confirmed live against kitty's own
vt-parser.c that it has no DECSLRM/DECLRMM support at all (CSI Ps r is
DECSTBM only; parameterized CSI s falls through to "Unknown CSI s
sequence"), and forcing margins on produced observable corruption when
scrolling a non-full-width pane, since kitty silently ignores the
column restriction tmux believes is protecting the rest of the screen.

tty-features.c

@@ -717,6 +717,7 @@ tty_default_features(struct client *c, const char *name, u_int version)
 			      "extkeys,"
 			      "focus,"
 			      "hyperlinks,"
+			      "imagescroll,"
 			      "osc7,"
 			      "sync,"
 			      "usstyle,"