vim-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.

5409f5d8c9

Omit source_nextline() and eval_next_non_blank(): Vim9 script only.

N/A patches for version.c:

vim-patch:8.2.1048: build failure without the eval feature

Problem:    Build failure without the eval feature.
Solution:   Add dummy typedef.

9d40c63c7d

vim-patch:8.2.1052: build failure with older compilers

Problem:    Build failure with older compilers.
Solution:   Move declaration to start of block.

7acde51832

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.h

@@ -266,11 +266,23 @@ typedef int (*ex_unletlock_callback)(lval_T *, char *, exarg_T *, int);
 // Used for checking if local variables or arguments used in a lambda.
 extern bool *eval_lavars_used;
 
+/// 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;
+
 /// Flag for expression evaluation.
 enum {
   EVAL_EVALUATE = 1,  ///< when missing don't actually evaluate
 };
 
+/// Passed to an eval() function to enable evaluation.
+EXTERN evalarg_T EVALARG_EVALUATE INIT(= { EVAL_EVALUATE, NULL });
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "eval.h.generated.h"
 #endif