Update Fantomas rules and require CI to pass Fantoma formatting

[abelbraaksma]

This updates the Fantomas rules w.r.t. to the Stroustrup formatting, which is not an experimental feature anymore, and now has changed the necessary config properties for the latest Fantomas versions.

This will require dotnet fantomas . to be run after non-formatted changes have been made.

[abelbraaksma]

There you go:

[abelbraaksma]

All done an dusted and green again. Merge time :). To any prospective committers, run this:

# run restore once
dotnet tool restore 

# run before you commit your changes
dotnet fantomas .