feat: add List.where method for filtering elements with predicate function

Implement a `where` method on the List class that takes a predicate function and returns a new list containing only elements for which the predicate returns true. The implementation iterates over the list, calls the predicate on each element, and collects matching elements into a new list. Includes test cases verifying filtering with both matching and non-matching predicates.

builtin/core.wren

@@ -16,4 +16,12 @@ class List {
     }
     return result
   }
+  
+  where (f) {
+    var result = []
+    for (element in this) {
+      if (f.call(element)) result.add(element)
+    }
+    return result
+  }
 }