patch 9.0.1250: cannot use an object method with :defer

Problem:    Cannot use an object method with :defer. (Ernie Rael)
Solution:   Find the object method and generate code to call it.
            (closes #11886)

src/vim9.h

@@ -122,6 +122,7 @@ typedef enum {
     ISN_NEWFUNC,    // create a global function from a lambda function
     ISN_DEF,	    // list functions
     ISN_DEFER,	    // :defer  argument count is isn_arg.number
+    ISN_DEFEROBJ,   // idem, function is an object method
 
     // expression operations
     ISN_JUMP,	    // jump if condition is matched isn_arg.jump