fix(element/scrollable): display and user interaction

Fix initial render to show scrollbar immediately if required. Show and
hide scrollbar correctly when content size or terminal size changes. Add
keybindings for scrolling similar to pager keybindings.

README.md

@@ -33,8 +33,6 @@ const zterm: *Dependency = b.dependency("zterm", .{
     .target = target,
     .optimize = optimize,
 });
-// ...
-exe.root_module.addImport("zterm", zterm.module("zterm"));
 ```
 
 ### Documentation