This is a package that is used internally to power Wrangler and Cloudflare Pages. It contains all the code that is shared between these clients (and possibly any other in the future e.g. the dashboard).
Code should be written so that it can be run in almost any environment (e.g. Deno, Node, browser, Workers), but note that there's currently a bit of weirdness here for asset-server, hence the environment-polyfills piece.