I had to increase the Node minimum support for this PR to work. Currently, sirv supported Node10+ but and node: was only added in Node 16 and back-ported to Node 14.18

The sibling Polka PR worked because the polka@next already had an exports map, which allowed/guaranteed anyone using ESM to get the node:-containing files, but all legacy/previous CommonJS usage to keep using files that did not have node: in it, which quickly broke many upstream CommonJS users.

This wasnt an option here, as sirv did not already have an exports map, so I couldnt guarantee that ESM users would get the new node: files (thus making this PR useless).

Why? The index.js files were built into build.mjs and build.js outputs. A build script could strip node:-prefixes in build.js (like the Polka PR), leaving them in build.mjs... but since sirv only has "module" mapping, which is now largely ignore/replaced by exports, it's unlikely that Svelte Kit / Deno users would ever see the build.mjs file.