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.

unicode-mode.c

@@ -34,10 +34,14 @@ unicode_mode_updated(struct terminal *term)
     if (term == NULL)
         return;
 
-    if (term->is_searching)
+#if defined(FOOT_HAVE_SCROLLBACK)
+    if (term->is_searching) {
         render_refresh_search(term);
-    else
+    } else
+#endif
+    {
         render_refresh(term);
+    }
 }
 
 void
@@ -57,10 +61,14 @@ unicode_mode_input(struct seat *seat, struct terminal *term,
                 term->unicode_mode.character, (int)chars, utf8);
 
         if (chars != (size_t)-1) {
-            if (term->is_searching)
+#if defined(FOOT_HAVE_SCROLLBACK)
+            if (term->is_searching) {
                 search_add_chars(term, utf8, chars);
-            else
+            } else
+#endif
+            {
                 term_to_slave(term, utf8, chars);
+            }
         }
 
         seat->kbd.last_shortcut_sym = sym;