Scala: now targeting 2.13 by default, with new flag for 2.12
Many thanks to @johanneshiry for this contribution!
This new release only impacts the Scala generation: tscfg now targets 2.13 by default, with a new --scala:2.12
flag as a mechanism for any need for compatibility with 2.12 as in previous releases. In general, the distinction is only relevant for configuration specs involving lists.
See #59 for more details.