patch 8.2.2090: Vim9: dict does not accept a key in quotes

Problem:    Vim9: dict does not accept a key in quotes.
Solution:   Recognize a key in single or double quotes.
diff --git a/src/proto/dict.pro b/src/proto/dict.pro
index 0ba8365..dbfdcdc 100644
--- a/src/proto/dict.pro
+++ b/src/proto/dict.pro
@@ -34,6 +34,7 @@
 varnumber_T dict_get_bool(dict_T *d, char_u *key, int def);
 char_u *dict2string(typval_T *tv, int copyID, int restore_copyID);
 char_u *skip_literal_key(char_u *key);
+char_u *get_literal_key(char_u **arg);
 int eval_dict(char_u **arg, typval_T *rettv, evalarg_T *evalarg, int literal);
 void dict_extend(dict_T *d1, dict_T *d2, char_u *action);
 dictitem_T *dict_lookup(hashitem_T *hi);