pw_console: ptpython repl, execution and display

Enables a ptpython based repl in the ReplPane. User repl code is
executed in it's own thread with stdout and stderr patched to
capture output.

Change-Id: I005481f1ecd6805ce9a74fbf57ff0f2317b9d2aa
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48960
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Joe Ethier <jethier@google.com>