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.

meson.build

@@ -90,6 +90,9 @@ add_project_arguments(
   (get_option('ime')
     ? ['-DFOOT_IME_ENABLED=1']
     : []) +
+  (get_option('scrollback')
+    ? ['-DFOOT_HAVE_SCROLLBACK']
+    : []) +
   (get_option('b_pgo') == 'use'
     ? ['-DFOOT_PGO_ENABLED=1']
     : []) +
@@ -443,6 +446,7 @@ summary(
     'Documentation': scdoc.found(),
     'Themes': get_option('themes'),
     'IME': get_option('ime'),
+    'Scrollback': get_option('scrollback'),
     'Grapheme clustering': utf8proc.found(),
     'utmp backend': utmp_backend,
     'utmp helper default path': utmp_default_helper_path,