LibCore+Everywhere: Move OpenMode out of IODevice

...and make it an enum class so people don't omit "OpenMode".

Userland/Shell/main.cpp

@@ -105,7 +105,7 @@ int main(int argc, char** argv)
     parser.parse(argc, argv);
 
     if (format) {
-        auto file = Core::File::open(format, Core::IODevice::ReadOnly);
+        auto file = Core::File::open(format, Core::OpenMode::ReadOnly);
         if (file.is_error()) {
             warnln("Error: {}", file.error());
             return 1;