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- [x] Add new / update outdated documentation

importScripts("./pyodide.js");

onmessage = async function (e) {
  try {
    const data = e.data;
    for (let key of Object.keys(data)) {
      if (key !== "python") {
        // Keys other than python must be arguments for the python script.
        // Set them on self, so that `from js import key` works.
        self[key] = data[key];
      }
    }

    if (!loadPyodide.inProgress) {
      self.pyodide = await loadPyodide();
    }
    await self.pyodide.loadPackagesFromImports(data.python);
    let results = await self.pyodide.runPythonAsync(data.python);
    self.postMessage({ results });
  } catch (e) {
    // if you prefer messages with the error
    self.postMessage({ error: e.message + "\n" + e.stack });
    // if you prefer onerror events
    // setTimeout(() => { throw err; });
  }
};

importScripts("./pyodide.js");

onmessage = async function (e) {
  try {
    const data = e.data;
    for (let key of Object.keys(data)) {
      if (key !== "python") {
        // Keys other than python must be arguments for the python script.
        // Set them on self, so that `from js import key` works.
        self[key] = data[key];
      }
    }

    if (!loadPyodide.inProgress) {
      self.pyodide = await loadPyodide();
    }
    await self.pyodide.loadPackagesFromImports(data.python);
    let results = await self.pyodide.runPythonAsync(data.python);
    self.postMessage({ results });
  } catch (e) {
    // if you prefer messages with the error
    self.postMessage({ error: e.message + "\n" + e.stack });
    // if you prefer onerror events
    // setTimeout(() => { throw err; });
  }
};
