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).