Handle SvelteKit breaking changes - build fails for versions >= next.208

[geoffrich]

sveltejs/kit#2580

• Import app via $server-build alias instead of hard-coded relative path
• Need to specify serverEntryPoint
• Simply import @sveltejs/kit/install-fetch directly instead of more complicated shim

Reference that PR for README updates as well (no more custom esbuild config).

[geoffrich]

Update: looks like sveltejs/kit#2931 is the correct PR to reference, which also includes changes to generate functions per route instead of a single monolith.

[geoffrich]

Also some changes to integrate w/ the request object -- path/query have been replaced with URL. See sveltejs/kit#3133

[geoffrich]

I made the updates to support v. next-208 and confirmed it works with the latest SvelteKit demo app. The linked PR did not include code-splitting API routes, since it seems like that's only implemented for the Netlify adapter and is marked as experimental. I'll look into implementing that feature when it's more stable.