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/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index a922543..de88ea7 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1509,6 +1509,15 @@
   assert_equal(123, d.key)
 enddef
 
+def Test_expr7_subscript()
+  let text = 'abcdef'
+  assert_equal('', text[-1])
+  assert_equal('a', text[0])
+  assert_equal('e', text[4])
+  assert_equal('f', text[5])
+  assert_equal('', text[6])
+enddef
+
 def Test_expr7_subscript_linebreak()
   let range = range(
   		3)