Lazy DevHtmlAsset chunk generation (#4679)
### Description
This fixes a perf regression introduced in the chunking refactor, where
we would be eagerly generating all assets from the web entry and
fallback sources.
### Testing Instructions
Benchmarks
[Next.js
PR](https://github.com/vercel/next.js/compare/alexkirsz/web-944-perf-regression?expand=1)
link WEB-944
