Add an optional build-time switch to disable scrollback support

This change introduces a new `scrollback` Meson build option (enabled
by default) that lets foot be compiled without scrollback history.
When the option is off, `FOOT_HAVE_SCROLLBACK` is left
undefined and the relevant code is excluded from the build,
producing a slimmer terminal for use cases that do not need it.

With scrollback disabled:

- The `[scrollback]` section in `foot.ini` and its key bindings
  become no-ops.
- In-terminal search is removed,
  along with the "pipe scrollback" action, the scrollback indicator,
  and their colors/overlays.
- IME hooks for the search box, mouse-wheel scrollback handling,
  and related `terminal` state are compiled out.
- Selection auto-scroll (the timer that scrolls the viewport while
  a drag-selection extends past the visible area) is removed,
  since with no scrollback there is nowhere to scroll to.
  The associated function declarations,
  `enum selection_scroll_direction`,
  the `auto_scroll` field on `terminal::selection`,
  and their init/teardown sites are all excluded from the build.

To keep action enumerations stable across build configurations,
two range markers (`BIND_ACTION_PIPE_FIRST` / `BIND_ACTION_PIPE_LAST`)
are introduced so that pipe-action handling does not depend on
`PIPE_SCROLLBACK` being present. The build summary and
`foot --version` now report `+scrollback` or `-scrollback`,
and the config tests have been updated to account for optional section.

wayland.c

@@ -2210,6 +2210,7 @@ wayl_win_destroy(struct wl_window *win)
         wl_surface_commit(win->render_timer.surface.surf);
     }
 
+#if defined(FOOT_HAVE_SCROLLBACK)
     if (win->scrollback_indicator.surface.surf != NULL) {
         wl_surface_attach(win->scrollback_indicator.surface.surf, NULL, 0, 0);
         wl_surface_commit(win->scrollback_indicator.surface.surf);
@@ -2220,6 +2221,7 @@ wayl_win_destroy(struct wl_window *win)
         wl_surface_attach(win->search.surface.surf, NULL, 0, 0);
         wl_surface_commit(win->search.surface.surf);
     }
+#endif /* FOOT_HAVE_SCROLLBACK */
 
     /* URLs */
     tll_foreach(win->urls, it) {
@@ -2253,13 +2255,17 @@ wayl_win_destroy(struct wl_window *win)
     render_wait_for_preapply_damage(term);
 
     csd_destroy(win);
+#if defined(FOOT_HAVE_SCROLLBACK)
     wayl_win_subsurface_destroy(&win->search);
     wayl_win_subsurface_destroy(&win->scrollback_indicator);
+#endif
     wayl_win_subsurface_destroy(&win->render_timer);
     wayl_win_subsurface_destroy(&win->overlay);
 
+#if defined(FOOT_HAVE_SCROLLBACK)
     shm_purge(term->render.chains.search);
     shm_purge(term->render.chains.scrollback_indicator);
+#endif
     shm_purge(term->render.chains.render_timer);
     shm_purge(term->render.chains.grid);
     shm_purge(term->render.chains.url);