add(element/scrollable): implement content provider

However this is only working for the same size as the parent container
(i.e. same `Size`). The `.resize` event for the `Container` of the
scrollable element needs to be the necessary and/or required size for
the contents (regardless of the screen viewport).

src/app.zig

@@ -38,7 +38,9 @@ pub fn App(comptime E: type) type {
     return struct {
         pub const Event = mergeTaggedUnions(event.SystemEvent, E);
         pub const Container = @import("container.zig").Container(Event);
-        pub const Element = @import("element.zig").Element(Event);
+        const element = @import("element.zig");
+        pub const Element = element.Element(Event);
+        pub const Scrollable = element.Scrollable(Event);
 
         queue: Queue(Event, 256),
         thread: ?std.Thread,