patch 8.2.1047: Vim9: script cannot use line continuation like :def function

Problem:    Vim9: script cannot use line continuation like in a :def function.
Solution:   Pass the getline function pointer to the eval() functions.  Use it
            for addition and multiplication operators.

src/globals.h

@@ -1880,6 +1880,9 @@ EXTERN char windowsVersion[20] INIT(= {0});
 
 // Used for lv_first in a non-materialized range() list.
 EXTERN listitem_T range_list_item;
+
+// Passed to an eval() function to enable evaluation.
+EXTERN evalarg_T EVALARG_EVALUATE INIT2(EVAL_EVALUATE, NULL);
 #endif
 
 #ifdef MSWIN