Add support for `default_account`

[milkyklim]

Describe

With such settings as default_ecosystem and default_network would be useful to set default_account in .yaml, too.

Specification

No response

Dependencies

No response

[linear]

APE-2013

[fubuloubu]

maybe default_sender: <ALIAS> to match the with accounts.use_sender():
https://docs.apeworx.io/ape/latest/userguides/accounts.html#id1

other question: would this just be for scripting use? or over the entire session of using ape?