add API for "package dir"

See https://github.com/nodejs/node/issues/33460.

tl;dr: it turns out that there's a number of tools that need to be able to locate a package's root directory, and with "exports", they can no longer rely on package/package.json working, nor package/.

If we could provide this API, then CJS could path.join it, and ESM could use URL, to get whatever file the tool needs to fs.readFile.

Adding this new API would bypass questions about "should package.json be an implicit part of a package's API", as well as avoid reliance on ESM-only or CJS-only mechanisms. By providing the package root dir rather than its package.json, we would not be encouraging or discouraging any patterns of "where to store metadata" - instead, we'd be correctly leaving that choice up to userland.

Example solution: module.packageDir(specifier) and module.packageDirSync(specifier) (sync access is critical for many use cases).