This is what I personally derived from a lot of sources as my starter for dotnet+svelte apps
Consists of the default svelte app template and a preconfigured dotnet project, along with a matching rollup config to run.
To use, clone/degit this repo (or click "Use Template"), npm install, then npm run dev
npx degit sveltejs/template <DIR>
cd <DIR>
dotnet new web --no-https
mv src svelte-app
mv public wwwroot
Then follow the steps here to fix
rollup.config.js and Startup.cs