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- [x] Add new / update outdated documentation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- [x] Add new / update outdated documentation

---
name: Code improvement
about: Suggest a code improvement, i.e. refactoring, deprecation, etc.
title: ""
labels:
assignees: ""
---

## Proposed refactoring or deprecation

<!-- A clear and concise description of the code improvement -->

### Motivation

<!-- Please outline the motivation for the proposal. If this is related to another GitHub issue, please link here too -->

### Pitch

<!-- A clear and concise description of what you want to happen. -->

### Additional context

<!-- Add any other context or screenshots here. -->

---
name: Code improvement
about: Suggest a code improvement, i.e. refactoring, deprecation, etc.
title: ""
labels:
assignees: ""
---

## Proposed refactoring or deprecation

<!-- A clear and concise description of the code improvement -->

### Motivation

<!-- Please outline the motivation for the proposal. If this is related to another GitHub issue, please link here too -->

### Pitch

<!-- A clear and concise description of what you want to happen. -->

### Additional context

<!-- Add any other context or screenshots here. -->
