feat(api): implement nvim_{add,del}_user_command

Add support for adding and removing custom user commands with the Nvim
API.

src/nvim/lua/executor.h

@@ -7,6 +7,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/eval/typval.h"
 #include "nvim/ex_cmds_defs.h"
+#include "nvim/ex_docmd.h"
 #include "nvim/func_attr.h"
 #include "nvim/lua/converter.h"