patch 8.2.1719: Vim9: no error if comma is missing in between arguments

Problem:    Vim9: no error if comma is missing in between arguments.
Solution:   Give an error message.

Author: brammool

src/errors.h

@@ -268,4 +268,6 @@ EXTERN char e_cannot_change_dict_item[]
 	INIT(= N_("E1121: Cannot change dict item"));
 EXTERN char e_variable_is_locked_str[]
 	INIT(= N_("E1122: Variable is locked: %s"));
+EXTERN char e_missing_comma_before_argument_str[]
+	INIT(= N_("E1123: Missing comma before argument: %s"));
 #endif

src/testdir/test_vim9_expr.vim

@@ -2112,6 +2112,7 @@ def Test_expr7_call()
    "vim9script",
    "let x = substitute ('x', 'x', 'x', 'x')"
    ], 'E121:', 2)
+  CheckDefFailure(["let Ref = function('len' [1, 2])"], 'E1123:', 1)
 
   let auto_lines =<< trim END
       def g:some#func(): string

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1719,
 /**/
     1718,
 /**/

src/vim9compile.c

@@ -2290,6 +2290,7 @@ compile_arguments(char_u **arg, cctx_T *cctx, int *argcount)
 {
     char_u  *p = *arg;
     char_u  *whitep = *arg;
+    int	    must_end = FALSE;
 
     for (;;)
     {
@@ -2300,6 +2301,11 @@ compile_arguments(char_u **arg, cctx_T *cctx, int *argcount)
 	    *arg = p + 1;
 	    return OK;
 	}
+	if (must_end)
+	{
+	    semsg(_(e_missing_comma_before_argument_str), p);
+	    return FAIL;
+	}
 
 	if (compile_expr0(&p, cctx) == FAIL)
 	    return FAIL;
@@ -2316,6 +2322,8 @@ compile_arguments(char_u **arg, cctx_T *cctx, int *argcount)
 	    if (*p != NUL && !VIM_ISWHITE(*p))
 		semsg(_(e_white_space_required_after_str), ",");
 	}
+	else
+	    must_end = TRUE;
 	whitep = p;
 	p = skipwhite(p);
     }