fix: allow empty range subscripts [0..-1] and [0...0] on empty lists

Add special case in list_subscript native to return a new empty list when
the source list is empty and the requested range is [0..-1] (inclusive) or
[0...0] (exclusive). This enables using list[0..-1] as a universal copy
idiom that works even for empty lists. Update the List.+ operator to use
this idiom instead of manual iteration. Add test cases for both range
forms on empty lists.

builtin/core.wren

@@ -10,10 +10,7 @@ class List {
   }
 
   + that {
-    var result = []
-    for (element in this) {
-      result.add(element)
-    }
+    var result = this[0..-1]
     for (element in that) {
       result.add(element)
     }