Add dependency fallbacks
meson: Add fallback for pcre2 and xml2 When building on Windows those 2 dependencies are usually not available, but Meson can build them as subproject. Simply run 'meson wrap install pcre2' and 'meson wrap install libxml2' gtk4 and all its dependencies can also be built as subproject if adding a gtk4.wrap file. With those gtksourceview and all its deps can fully build on Windows with MSVC.
meson: Use a feature option for introspection This copy the same behaviour as in gtk4 and all its dependencies. It fix the build for example on Windows where gobject-introspection is not usually available.