patch 8.2.1306: checking for first character of dict key is inconsistent

Problem:    Checking for first character of dict key is inconsistent.
Solution:   Add eval_isdictc(). (closes #6546)
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index 5d04e01..a528d3e 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -58,6 +58,7 @@
 char_u *find_name_end(char_u *arg, char_u **expr_start, char_u **expr_end, int flags);
 int eval_isnamec(int c);
 int eval_isnamec1(int c);
+int eval_isdictc(int c);
 int handle_subscript(char_u **arg, typval_T *rettv, evalarg_T *evalarg, int verbose);
 int item_copy(typval_T *from, typval_T *to, int deep, int copyID);
 void echo_one(typval_T *rettv, int with_space, int *atstart, int *needclr);