This is the mirror image of #2170.

### Checklists

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     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
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- [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- [x] Add new / update outdated documentation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- [x] Add new / update outdated documentation

[metadata]
name = pyodide-build
version = 0.20.0dev0
author = Pyodide developers
description = "Tools for building Pyodide"
long_description = file: README.md
long_description_content_type = text/markdown
url = https://github.com/pyodide/pyodide
project_urls =
    Bug Tracker = https://github.com/pyodide/pyodide/issues
    Documentation = https://pyodide.org/en/stable/
classifiers =
    Programming Language :: Python :: 3
    License :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)
    Operating System :: OS Independent

[options]
package_dir =
    = .
packages = find:
python_requires = >=3.8
install_requires =
    pyyaml
    cython<3.0
[options.entry_points]
console_scripts =
    pyodide-build = pyodide_build.__main__:main

[options.packages.find]
where = .

[metadata]
name = pyodide-build
version = 0.20.0dev0
author = Pyodide developers
description = "Tools for building Pyodide"
long_description = file: README.md
long_description_content_type = text/markdown
url = https://github.com/pyodide/pyodide
project_urls =
    Bug Tracker = https://github.com/pyodide/pyodide/issues
    Documentation = https://pyodide.org/en/stable/
classifiers =
    Programming Language :: Python :: 3
    License :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)
    Operating System :: OS Independent

[options]
package_dir =
    = .
packages = find:
python_requires = >=3.8
install_requires =
    pyyaml
    cython<3.0
[options.entry_points]
console_scripts =
    pyodide-build = pyodide_build.__main__:main

[options.packages.find]
where = .
