Support more granular page compilations

[bitspittle]

Per file compilation is now available with kotlin 1.9.20 Would be interesting to see if we can get webpack (or vite?) to use this to bundle pages separately

Some ideas with this as a foundation:

• something like next.js prefetching when you visit a page, so individual pages are smaller but you can still get the effect of instant navigation if you click on something that was already fetched.
• Break up a site into several subsections, and group pages together. This could help a project scale better if you can put common pages in one group and leave uncommon pages in another group.
• Have all the pages be grouped into one bundle but separate out the kotlin stdlib and kobweb parts. This might play well with caching, where a user can add new pages over time, but browsers will still have an old valid copy of the kobweb stdlib js output around.