More automation

[lkubb]

• Tries to execute the first steps automatically after copy
• Also updates the virtualenv and runs pre-commit after update
• Does not recreate deleted boilerplate anymore (yay!)
• Also adds a dev env init script for contributors that just clone
• Adds a .envrc file for direnv that calls the above script and sources the venv.
• Adds test runs on Windows and macOS

Uses some hacky workarounds while _copier_conf.operation is not available.

Fixes: https://github.com/salt-extensions/salt-extension-copier/issues/41 Fixes: https://github.com/salt-extensions/salt-extension-copier/issues/45