spike(#3347): sticky header utility classes

[bdfranck]

This spike explores one sticky header idea: what if we used CSS utility classes?

<div class="scroll-container">
  <div class="sticky">
    <goab-container>This is sticky</goab-container>
  </div>
  <!-- tons of content goes here -->
</div>

Benefits

• Lightweight & high performance
• Only uses CSS
• SImple and flexible

Drawbacks

• Goes against our current Svelte component pattern
• Teams may have trouble implementing it if they don't know how position: sticky works
• Still needs JS to add shadows

Would more simply utility classes like this be useful for simple layout assistance?