patch 8.2.1463: Vim9: list slice not supported yet

Problem:    Vim9: list slice not supported yet.
Solution:   Add support for list slicing.
diff --git a/src/vim9.h b/src/vim9.h
index 5494486..ce92d03 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -119,6 +119,7 @@
     ISN_STRINDEX,   // [expr] string index
     ISN_STRSLICE,   // [expr:expr] string slice
     ISN_LISTINDEX,  // [expr] list index
+    ISN_LISTSLICE,  // [expr:expr] list slice
     ISN_SLICE,	    // drop isn_arg.number items from start of list
     ISN_GETITEM,    // push list item, isn_arg.number is the index
     ISN_MEMBER,	    // dict[member]