This is the mirror image of #2170.

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- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
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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- [x] Add new / update outdated documentation

commit 7352fc4a977a3414cf3194faf6ef990638a6329c
Author: Roman Yurchak <rth.yurchak@gmail.com>
Date:   Sat Feb 27 11:13:29 2021 +0100

    disable-blas-detection.patch

BLAS and LAPACK aren't available on host because we only cross compile these
libraries (see CLAPACK/meta.yaml). Scipy tries to detect these libraries and
if it fails to find them errors out the build. But we have them installed
correctly for our target, so we just disable this detection mechanism.

diff --git a/setup.py b/setup.py
index 0e11de60f..8c28c613d 100755
--- a/setup.py
+++ b/setup.py
@@ -484,7 +484,7 @@ def configuration(parent_package='', top_path=None):
 
     lapack_opt = get_info('lapack_opt')
 
-    if not lapack_opt:
+    if not lapack_opt and False:
         if sys.platform == "darwin":
             msg = ('No BLAS/LAPACK libraries found. '
                    'Note: Accelerate is no longer supported.')

commit 7352fc4a977a3414cf3194faf6ef990638a6329c
Author: Roman Yurchak <rth.yurchak@gmail.com>
Date:   Sat Feb 27 11:13:29 2021 +0100

    disable-blas-detection.patch

BLAS and LAPACK aren't available on host because we only cross compile these
libraries (see CLAPACK/meta.yaml). Scipy tries to detect these libraries and
if it fails to find them errors out the build. But we have them installed
correctly for our target, so we just disable this detection mechanism.

diff --git a/setup.py b/setup.py
index 0e11de60f..8c28c613d 100755
--- a/setup.py
+++ b/setup.py
@@ -484,7 +484,7 @@ def configuration(parent_package='', top_path=None):
 
     lapack_opt = get_info('lapack_opt')
 
-    if not lapack_opt:
+    if not lapack_opt and False:
         if sys.platform == "darwin":
             msg = ('No BLAS/LAPACK libraries found. '
                    'Note: Accelerate is no longer supported.')
