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.