fs-constants: Cross-Environment FS Constants

The `fs-constants` package provides a unified and compatible way to access file system constants (like `O_RDONLY`, `S_IFMT`) across both Node.js and browser environments. It transparently handles the historical divergence and deprecation in Node.js, where `require('constants')` was replaced by `require('fs').constants`. For Node.js, it uses `require('fs').constants`, and for browser environments, it leverages `require('constants')`, which bundlers often polyfill or provide. Currently at version 1.0.0, it is a stable, low-maintenance utility designed to abstract away environment-specific constant imports, enabling more portable code for file system operations. Its primary differentiator is resolving this specific Node.js API evolution and ensuring browser compatibility for `fs` flags without complex conditional logic in application code.