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.

ime.c

@@ -177,10 +177,14 @@ done(void *data, struct zwp_text_input_v3 *zwp_text_input_v3,
         ime_reset_preedit(seat);
 
         if (term != NULL) {
-            if (term->is_searching)
+#if defined(FOOT_HAVE_SCROLLBACK)
+            if (term->is_searching) {
                 render_refresh_search(term);
-            else
+            } else
+#endif
+            {
                 render_refresh(term);
+            }
         }
     }
 
@@ -198,11 +202,15 @@ done(void *data, struct zwp_text_input_v3 *zwp_text_input_v3,
         size_t len = strlen(text);
 
         if (term != NULL) {
+#if defined(FOOT_HAVE_SCROLLBACK)
             if (term->is_searching) {
                 search_add_chars(term, text, len);
                 render_refresh_search(term);
             } else
+#endif
+            {
                 term_to_slave(term, text, len);
+            }
         }
         ime_reset_pending_commit(seat);
     }
@@ -367,10 +375,14 @@ done(void *data, struct zwp_text_input_v3 *zwp_text_input_v3,
     ime_reset_pending_preedit(seat);
 
     if (term != NULL) {
-        if (term->is_searching)
+#if defined(FOOT_HAVE_SCROLLBACK)
+        if (term->is_searching) {
             render_refresh_search(term);
-        else
+        } else
+#endif
+        {
             render_refresh(term);
+        }
     }
 }
 
@@ -473,9 +485,11 @@ ime_update_cursor_rect(struct seat *seat)
     if (seat->ime.preedit.cells != NULL)
         goto update;
 
+#if defined(FOOT_HAVE_SCROLLBACK)
     /* Set in render_search_box() */
     if (term->is_searching)
         goto update;
+#endif
 
     int x, y, width, height;
     int col = term->grid->cursor.point.col;