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- [x] Add new / update outdated documentation

# Pyodide packages

This folder contains the list of packages built in pyodide.

Packages are built with the Python build system
using `pyodide_build/buildall.py` and `pyodide_build/buildpkg.py` scripts.
See the [documentation](https://pyodide.org/en/stable/development/new-packages.html) for more details.

# Pyodide packages

This folder contains the list of packages built in pyodide.

Packages are built with the Python build system
using `pyodide_build/buildall.py` and `pyodide_build/buildpkg.py` scripts.
See the [documentation](https://pyodide.org/en/stable/development/new-packages.html) for more details.
