patch 9.0.1041: cannot define a method in a class

Problem:    Cannot define a method in a class.
Solution:   Implement defining an object method.  Make calling an object
            method work.

src/vim9.h

@@ -33,6 +33,7 @@ typedef enum {
     ISN_SOURCE,	    // source autoload script, isn_arg.number is the script ID
     ISN_INSTR,	    // instructions compiled from expression
     ISN_CONSTRUCT,  // construct an object, using contstruct_T
+    ISN_OBJ_MEMBER, // object member, index is isn_arg.number
 
     // get and set variables
     ISN_LOAD,	    // push local variable isn_arg.number