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/structs.h

@@ -1746,6 +1746,19 @@ typedef struct
 # endif
 } scriptitem_T;
 
+// Struct passed through eval() functions.
+// See EVALARG_EVALUATE for a fixed value with eval_flags set to EVAL_EVALUATE.
+typedef struct {
+    int		eval_flags;	// EVAL_ flag values below
+
+    // copied from exarg_T when "getline" is "getsourceline". Can be NULL.
+    void	*eval_cookie;	// argument for getline()
+} evalarg_T;
+
+// Flags for expression evaluation.
+#define EVAL_EVALUATE	    1	    // when missing don't actually evaluate
+#define EVAL_CONSTANT	    2	    // when not a constant return FAIL
+
 # ifdef FEAT_PROFILE
 /*
  * Struct used in sn_prl_ga for every line of a script.