Implement `read-xlsx`

Read XLSX files. `@stdlib/fs/read-xlsx`.

This package should only focus on xlsx files. Other file formats should be addressed in separate packages.

Start by locating the @stdlib/fs/read-xlsx package entry point and reviewing how existing filesystem readers are organized. Confirm the package handles only XLSX files, and identify the project’s test location and expected reader behavior before implementation. Done means the package reads XLSX files without expanding into other formats.

Written by the indexing model from the issue text.