Export PATH and ERRNO_CODES from Emscripten. ### Description

Fixes #2577 by exporting `PATH` and `ERRNO_CODES` from Emscripten

### Checklists

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- [x] Add new / update outdated documentation

--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -38,7 +38,7 @@ plugin_meta_data = {}
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }
 
--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -38,7 +38,7 @@ plugin_meta_data = {}
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }
 