feat: add an eta/core subpath export

[panva]

This PR adds a subpath export so that eta/core can be imported/required in runtimes without "node:fs", i.e. CF Workers.

[bgub]

Looks good to me, but shouldn't the "browser" in the export already make it available to browsers?

[panva]

For when you don't know the target runtime but need to make sure no "fs" is required I guess?

It happens with wrangler, turbopack that when they get a whiff of a node builtin they either start including subpar polyfills or outright don't allow your code to bundle.