Fix Scalafmt

[andreisilviudragnea]

• Run Scalafmt in validateOnly mode when building
• Use Scalafmt defaults

Fixes #29.

[evis]

Two issues here:

1. can't simply run mvn scalafmt:format in order to format code locally;
2. can't compile unformatted files.

I suggest to remove validateOnly and don't run scalafmt on format goal. Instead, we should check formatting on CI using github action, similar to https://github.com/evis/scalafix-maven-plugin/blob/master/.github/workflows/scalafix.yml.

This way, the issues will be solved, and formatting will still be checked on CI.