Export PATH and ERRNO_CODES from Emscripten. ### Description

Fixes #2577 by exporting `PATH` and `ERRNO_CODES` from Emscripten

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).
