feat(element/input): text input element implementation

Moved implementation from example/input as a standalone `Element`
implementation, which is directly used by the example instead.

The provided argument is the `App.Event`'s event that should be
triggered on acceptance for the contents of the Input `Element`.
Currently only `[]u21` strings are supported, but in the future also
`[]u8` strings shall be supported and automatically converted when
pushed as an `App.Event` into the app's queue.

src/app.zig

@@ -418,6 +418,7 @@ pub fn App(comptime E: type) type {
         pub const Element = element.Element(Event);
         pub const Alignment = element.Alignment(Event);
         pub const Scrollable = element.Scrollable(Event);
+        pub const Input = element.Input(Event, Queue);
         pub const Queue = queue.Queue(Event, 256);
     };
 }