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/zterm.zig

@@ -19,6 +19,7 @@ pub const Layout = container.Layout;
 pub const Cell = @import("cell.zig");
 pub const Color = color.Color;
 pub const Key = @import("key.zig");
+pub const Position = size.Position;
 pub const Size = size.Size;
 pub const Style = @import("style.zig");