patch 8.2.3423: Vim9: list += list creates a new list in :def function

Problem:    Vim9: list += list creates a new list in :def function.
Solution:   Append to the existing list.

src/structs.h

@@ -4106,6 +4106,9 @@ typedef enum
     EXPR_MULT,		// *
     EXPR_DIV,		// /
     EXPR_REM,		// %
+    // used with ISN_ADDLIST
+    EXPR_COPY,		// create new list
+    EXPR_APPEND,	// append to first list
 } exprtype_T;
 
 /*