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

Problem:    Vim9: cannot use a range with :unlet.
Solution:   Implement ISN_UNLETRANGE.

src/list.c

@@ -530,6 +530,26 @@ list_find_str(list_T *l, long idx)
     return tv_get_string(&li->li_tv);
 }
 
+/*
+ * Like list_find() but when a negative index is used that is not found use
+ * zero and set "idx" to zero.  Used for first index of a range.
+ */
+    listitem_T *
+list_find_index(list_T *l, long *idx)
+{
+    listitem_T *li = list_find(l, *idx);
+
+    if (li == NULL)
+    {
+	if (*idx < 0)
+	{
+	    *idx = 0;
+	    li = list_find(l, *idx);
+	}
+    }
+    return li;
+}
+
 /*
  * Locate "item" list "l" and return its index.
  * Returns -1 when "item" is not in the list.