This is the mirror image of #2170.

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

### Checklists

<!-- Note on checklists:
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     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- [x] Add new / update outdated documentation

#undef HAVE_EPOLL
#undef HAVE_EPOLL_CREATE1
#undef HAVE_LINUX_VM_SOCKETS_H
#undef HAVE_SOCKETPAIR
#undef HAVE_UTIMENSAT
#undef HAVE_SIGACTION

/* Untested syscalls in emscripten */
#undef HAVE_OPENAT
#undef HAVE_MKDIRAT
#undef HAVE_FCHOWNAT
#undef HAVE_RENAMEAT
#undef HAVE_LINKAT
#undef HAVE_SYMLINKAT
#undef HAVE_READLINKAT
#undef HAVE_FCHMODAT
#undef HAVE_DUP3

/* Syscalls not implemented in emscripten */
#undef HAVE_PREADV
#undef HAVE_PWRITEV
#undef HAVE_PIPE2
#undef HAVE_NICE

/* Syscalls that resulted in a segfault */
#undef HAVE_UTIMENSAT
#undef HAVE_SYS_SOCKET_H
#undef HAVE_SYS_IOCTL_H

/* Unsupported functionality */
#undef HAVE_PTHREAD_H

#define CONFIG_32
#define ANSI

#undef HAVE_EPOLL
#undef HAVE_EPOLL_CREATE1
#undef HAVE_LINUX_VM_SOCKETS_H
#undef HAVE_SOCKETPAIR
#undef HAVE_UTIMENSAT
#undef HAVE_SIGACTION

/* Untested syscalls in emscripten */
#undef HAVE_OPENAT
#undef HAVE_MKDIRAT
#undef HAVE_FCHOWNAT
#undef HAVE_RENAMEAT
#undef HAVE_LINKAT
#undef HAVE_SYMLINKAT
#undef HAVE_READLINKAT
#undef HAVE_FCHMODAT
#undef HAVE_DUP3

/* Syscalls not implemented in emscripten */
#undef HAVE_PREADV
#undef HAVE_PWRITEV
#undef HAVE_PIPE2
#undef HAVE_NICE

/* Syscalls that resulted in a segfault */
#undef HAVE_UTIMENSAT
#undef HAVE_SYS_SOCKET_H
#undef HAVE_SYS_IOCTL_H

/* Unsupported functionality */
#undef HAVE_PTHREAD_H

#define CONFIG_32
#define ANSI
