patch 8.2.1462: Vim9: string slice not supported yet

Problem:    Vim9: string slice not supported yet.
Solution:   Add support for string slicing.
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index ef89322..d901cb0 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -971,7 +971,7 @@
   assert_equal('aabb', ConcatString())
 enddef
 
-def StringIndex(): number
+def StringIndex(): string
   let s = "abcd"
   let res = s[1]
   return res