Add assets function to page

[yadunut]

I added a function to get the list of assets that belong to a page. I didn't see something that could do this in the docs but the documentation in superhtml seems to reference .assets.

I was using this to automatically insert images I had in a folder into a page. I realized I might be able do something similar by generating a JSON/ziggy file with the image paths using a zig build step(maybe, I haven't tried this yet) so I'm unsure if this PR is warranted but I hope it might be useful to somebody.

This is my first time writing some proper Zig code, so would love any critiques.

Use Case

<div :loop="$page.assets()">
  <img src="$loop.it.link()" alt="Page asset">
</div>

This allows for dynamic gallery generation without maintaining separate asset lists.