Move harfbuzz:icu=disabled to meson.build

With this, no additional flags except -Ddefault_library=static are
needed to configure Aegisub on Windows.

Author: arch1t3cht

.github/workflows/ci.yml

@@ -27,7 +27,6 @@ jobs:
               -Dfreetype2:harfbuzz=disabled 
               -Dharfbuzz:freetype=disabled 
               -Dharfbuzz:cairo=disabled 
-              -Dharfbuzz:icu=disabled 
               -Dharfbuzz:glib=disabled 
               -Dharfbuzz:gobject=disabled
               -Dharfbuzz:tests=disabled

meson.build

@@ -1,7 +1,7 @@
 project('Aegisub', ['c', 'cpp'],
         license: 'BSD-3-Clause',
         meson_version: '>=0.57.0',
-        default_options: ['cpp_std=c++20', 'buildtype=debugoptimized'],
+        default_options: ['cpp_std=c++20', 'buildtype=debugoptimized', 'harfbuzz:icu=disabled'],
         version: '3.4.0')
 
 cmake = import('cmake')