build: Publish source jars

Context

Fixes #52 (closed).

What has changed?

Main Changes

• Trivial modification to convention plugin to have published source jars

Others

• Set encoding to UTF-8 to avoid issues on CI machine:
  • Default charset on CI machine is ASCII but Javadoc has characters like ß that I'd like to keep
  • Not strictly necessary now since we don't publish Javadoc jar but will be necessary when we publish Javadoc somewhere.
• Enable parallel publishing again, issue with parallel (and sequential actually) Maven publishing is supposed to be fixed on Gitlab's side.