meson: default to buildtype debugoptimized

meson.build

 project('Aegisub', ['c', 'cpp'],
         license: 'BSD-3-Clause',
         meson_version: '>=0.55.0',
-        default_options: ['cpp_std=c++11', 'b_lto=true'],
+        default_options: ['cpp_std=c++11', 'b_lto=true', 'buildtype=debugoptimized'],
         version: '3.2.2')
 
 cmake = import('cmake')