LibIMAP: Support for APPEND

Author: https://github.com/X-yl
Commit: 16995dc3d9
Pull-request: https://github.com/SerenityOS/serenity/pull/7735
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/MaxWipfli
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibIMAP/Objects.h

@@ -17,6 +17,7 @@
 
 namespace IMAP {
 enum class CommandType {
+    Append,
     Capability,
     Copy,
     Create,