patch 8.2.2533: Vim9: cannot use a range with :unlet

Problem:    Vim9: cannot use a range with :unlet.
Solution:   Implement ISN_UNLETRANGE.
diff --git a/src/vim9.h b/src/vim9.h
index 1cf4b3c..ca8e64d 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -61,6 +61,7 @@
     ISN_UNLET,		// unlet variable isn_arg.unlet.ul_name
     ISN_UNLETENV,	// unlet environment variable isn_arg.unlet.ul_name
     ISN_UNLETINDEX,	// unlet item of list or dict
+    ISN_UNLETRANGE,	// unlet items of list
 
     ISN_LOCKCONST,	// lock constant value