Backend centric event definition

[cruxcode]

The backend currently have three functions that are evoked at different points in the lifecycle of the app.

• handle_init_state - Evoked during build time.
• handle_page_request - Evoked on each request from client.
• handle_event - Evoked upon a user interaction in the browser.

A developer might want to hook into events like load, scroll & IntersectionObserver etc.