New web component: <throb-ber>

Adds a small rainbow gradient overlay until all of the images nested inside have finished loading. Works best with expensive dynamically generated images (like dynamic screenshot images).

📮 Blog post: https://www.zachleat.com/web/throbber/
🖼️ Demo: https://zachleat.github.io/throbber/demo.html
⚙️ Source code: https://github.com/zachleat/throbber