Make `publishingType` more typesafe

[martinbonnin]

Use sealed classes/interfaces instead of a String.

See #169 (comment) for more details.

API compatibility and migration path TBD.

[hfhbd]

Enum would be better for Groovy support (if you care :)

[martinbonnin]

Right. FWIW, I tried to use an enum in tapmoc and the ergonomics weren't great because of an issue in IJ autocomplete. I like type safety in general but I don't like the current experience in Gradle build scripts.