Add `newtype` topic for better library discoverability

[neko-kai]

I finally found this lib which seems far better than alternatives https://github.com/alexknvl/newtypes , https://github.com/vitorsvieira/dilate , https://github.com/rudogma/scala-supertagged – but i found it accidentally even though I searched actively for ways to get newtypes in scala, adding a newtype, tagged, tagged-types tags might make it easier to discover the lib

[carymrobbins]

@kaishh Great idea! I added the following topics, feel free to let me know if some others might be useful as well.

• newtype
• scala
• tagged
• tagged-types