patch 8.2.2033: Vim9: :def without argument gives compilation error

Problem:    Vim9: :def without argument gives compilation error.
Solution:   Add the DEF instruction. (closes #7344)

src/vim9.h

@@ -82,6 +82,7 @@ typedef enum {
     ISN_RETURN,	    // return, result is on top of stack
     ISN_FUNCREF,    // push a function ref to dfunc isn_arg.funcref
     ISN_NEWFUNC,    // create a global function from a lambda function
+    ISN_DEF,	    // list functions
 
     // expression operations
     ISN_JUMP,	    // jump if condition is matched isn_arg.jump