patch 8.2.1247: Vim9: cannot index a character in a string

Problem:    Vim9: cannot index a character in a string.
Solution:   Add ISN_STRINDEX instruction. (closes #6478)
diff --git a/src/version.c b/src/version.c
index e1df007..559d864 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1247,
+/**/
     1246,
 /**/
     1245,