feat: add contains method to List and update Set example to new operator syntax

Add a `contains(element)` method to the `List` class in the core library,
enabling linear search for an element across all list items. Update the
`example/set.wren` file to use the new Wren operator syntax for `|`, `+`,
`&`, and `-` methods, replacing old function-style parameter definitions
with block-style closures. Also fix trailing whitespace throughout the
Set example and embed the new `contains` method into the C source string
in `src/wren_core.c` for the built-in List class.

builtin/core.wren

@@ -41,6 +41,15 @@ class List is Sequence {
     }
     return result
   }
+
+  contains(element) {
+    for (item in this) {
+      if (element == item) {
+        return true
+      }
+    }
+    return false
+  }
 }
 
 class Range is Sequence {}