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/vim9.h b/src/vim9.h
index 8789414..5494486 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -117,6 +117,7 @@
     // expression operations
     ISN_CONCAT,
     ISN_STRINDEX,   // [expr] string index
+    ISN_STRSLICE,   // [expr:expr] string slice
     ISN_LISTINDEX,  // [expr] list index
     ISN_SLICE,	    // drop isn_arg.number items from start of list
     ISN_GETITEM,    // push list item, isn_arg.number is the index