patch 8.2.2728: special key names don't work if 'isident' is cleared

Problem:    Special key names don't work if 'isident' is cleared.
Solution:   Add vim_isNormalIDc() and use it for special key names.
            (closes #2389)

src/charset.c

@@ -834,6 +834,16 @@ vim_isIDc(int c)
     return (c > 0 && c < 0x100 && (g_chartab[c] & CT_ID_CHAR));
 }
 
+/*
+ * Like vim_isIDc() but not using the 'isident' option: letters, numbers and
+ * underscore.
+ */
+    int
+vim_isNormalIDc(int c)
+{
+    return ASCII_ISALNUM(c) || c == '_';
+}
+
 /*
  * return TRUE if 'c' is a keyword character: Letters and characters from
  * 'iskeyword' option for the current buffer.