This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [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- [x] Add new / update outdated documentation

Add `default_converter` argument to `to_js` and `toJs`. This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [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- [x] Add new / update outdated documentation

#ifndef PYTHON2JS_H
#define PYTHON2JS_H

/** Translate Python objects to JavaScript.
 */
// clang-format off
#define PY_SSIZE_T_CLEAN
#include "Python.h"
// clang-format on
#include "hiwire.h"

/**
 * Do a shallow conversion from python to JavaScript. Convert immutable types
 * with equivalent JavaScript immutable types, but all other types are proxied.
 */
JsRef
python2js(PyObject* x);

/**
 * Like python2js except in the handling of PyProxy creation.
 *
 * If proxies is NULL, will throw an error instead of creating a PyProxy.
 * Otherwise, proxies should be an Array and python2js_track_proxies will add
 * the proxy to the array if one is created.
 */
JsRef
python2js_track_proxies(PyObject* x, JsRef proxies);

/**
 * Convert a Python object to a JavaScript object, copying standard collections
 * into javascript down to specified depth
 * \param x The Python object
 * \param depth The maximum depth to copy
 * \param proxies If this is Null, will raise an error if we have no JavaScript
 *        conversion for a Python object. If not NULL, should be a JavaScript
 *        list. We will add all PyProxies created to the list.
 * \return The JavaScript object -- might be an Error object in the case of an
 *         exception.
 */
JsRef
python2js_with_depth(PyObject* x, int depth, JsRef proxies);

int
python2js_init(PyObject* core);

#endif /* PYTHON2JS_H */

#ifndef PYTHON2JS_H
#define PYTHON2JS_H

/** Translate Python objects to JavaScript.
 */
// clang-format off
#define PY_SSIZE_T_CLEAN
#include "Python.h"
// clang-format on
#include "hiwire.h"

/**
 * Do a shallow conversion from python to JavaScript. Convert immutable types
 * with equivalent JavaScript immutable types, but all other types are proxied.
 */
JsRef
python2js(PyObject* x);

/**
 * Like python2js except in the handling of PyProxy creation.
 *
 * If proxies is NULL, will throw an error instead of creating a PyProxy.
 * Otherwise, proxies should be an Array and python2js_track_proxies will add
 * the proxy to the array if one is created.
 */
JsRef
python2js_track_proxies(PyObject* x, JsRef proxies);

/**
 * Convert a Python object to a JavaScript object, copying standard collections
 * into javascript down to specified depth
 * \param x The Python object
 * \param depth The maximum depth to copy
 * \param proxies If this is Null, will raise an error if we have no JavaScript
 *        conversion for a Python object. If not NULL, should be a JavaScript
 *        list. We will add all PyProxies created to the list.
 * \return The JavaScript object -- might be an Error object in the case of an
 *         exception.
 */
JsRef
python2js_with_depth(PyObject* x, int depth, JsRef proxies);

/**
 * dict_converter should be a JavaScript function that converts an Iterable of
 * pairs into the desired JavaScript object. If dict_converter is NULL, we use
 * python2js_with_depth which converts dicts to Map (the default)
 */
JsRef
python2js_custom(PyObject* x,
                 int depth,
                 JsRef proxies,
                 JsRef dict_converter,
                 JsRef default_converter);

int
python2js_init(PyObject* core);

#endif /* PYTHON2JS_H */
