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/funcs.c

@@ -1550,7 +1550,7 @@ static void f_eval(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   }
 
   const char *const expr_start = s;
-  if (s == NULL || eval1((char **)&s, rettv, EVAL_EVALUATE) == FAIL) {
+  if (s == NULL || eval1((char **)&s, rettv, &EVALARG_EVALUATE) == FAIL) {
     if (expr_start != NULL && !aborting()) {
       semsg(_(e_invexpr2), expr_start);
     }