Replace SopelWrapper by Sopel in the code, and add code deprecation warning to prevent its future usage

[Exirel]

Part of the replacement of SopelWrapper with contextvars:

• Sopel should be used instead of SopelWrapper in the code
• improve testing tools so it is much easier to test with a "context sensitive instance" to ease future breaking change
• extensively document how to test with Sopel instead of SopelWrapper