Issue #16129: Add `Py_SetStandardStreamEncoding`

This new pre-initialization API allows embedding
applications like Blender to force a particular
encoding and error handler for the standard IO streams.

Also refactors Modules/_testembed.c to let us start
testing multiple embedding scenarios.

(Initial patch by Bastien Montagne)
diff --git a/Misc/ACKS b/Misc/ACKS
index 7be6f58..ab39bcf 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -872,6 +872,7 @@
 Florian Mladitsch
 Doug Moen
 The Dragon De Monsyne
+Bastien Montagne
 Skip Montanaro
 Peter Moody
 Paul Moore