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.

key-binding.c

@@ -111,7 +111,9 @@ key_binding_new_for_seat(struct key_binding_manager *mgr,
         struct key_set set = {
             .public = {
                 .key = tll_init(),
+#if defined(FOOT_HAVE_SCROLLBACK)
                 .search = tll_init(),
+#endif
                 .url = tll_init(),
                 .mouse = tll_init(),
             },
@@ -152,7 +154,9 @@ key_binding_new_for_conf(struct key_binding_manager *mgr,
         struct key_set set = {
             .public = {
                 .key = tll_init(),
+#if defined(FOOT_HAVE_SCROLLBACK)
                 .search = tll_init(),
+#endif
                 .url = tll_init(),
                 .mouse = tll_init(),
             },
@@ -531,6 +535,7 @@ convert_key_bindings(struct key_set *set)
     }
 }
 
+#if defined(FOOT_HAVE_SCROLLBACK)
 static void
 convert_search_bindings(struct key_set *set)
 {
@@ -541,6 +546,7 @@ convert_search_bindings(struct key_set *set)
         convert_key_binding(set, binding, &set->public.search);
     }
 }
+#endif /* FOOT_HAVE_SCROLLBACK */
 
 static void
 convert_url_bindings(struct key_set *set)
@@ -597,7 +603,9 @@ load_keymap(struct key_set *set)
     }
 
     convert_key_bindings(set);
+#if defined(FOOT_HAVE_SCROLLBACK)
     convert_search_bindings(set);
+#endif
     convert_url_bindings(set);
     convert_mouse_bindings(set);
 
@@ -638,7 +646,9 @@ static void NOINLINE
 unload_keymap(struct key_set *set)
 {
     key_bindings_destroy(&set->public.key);
+#if defined(FOOT_HAVE_SCROLLBACK)
     key_bindings_destroy(&set->public.search);
+#endif
     key_bindings_destroy(&set->public.url);
     key_bindings_destroy(&set->public.mouse);
     set->public.selection_overrides = 0;