feat: implement List concat operator + supporting Range and empty lists

Add `+` method to List class in core.wren that creates a new list by concatenating elements from both operands. Supports concatenation with Range objects and handles empty lists on either side. Includes comprehensive test cases in test/list/concat.wren covering list-list, list-range, and empty list combinations.

builtin/core.wren

@@ -8,4 +8,19 @@ class List {
     result = result + "]"
     return result
   }
+
+  + that {
+    var newList = []
+    if (this.count > 0) {
+      for (element in this) {
+        newList.add(element)
+      }
+    }
+    if (that is Range || that.count > 0) {
+      for (element in that) {
+        newList.add(element)
+      }
+    }
+    return newList
+  }
 }