Avoid warnings about unused values

[som-snytt]

Noticed at https://discord.com/channels/632150470000902164/632150470000902166/1115280559023861800

This commit experiments with turning on lints, which helps test the following lintable changes.

The case is explicitly () to avoid inferring a LUB of Any or AnyVal for the match, since that value is unused. The actual LUB is Unit, which naturally never warns.

Two expressions are annotated nowarn because they are not consumed.

One change is for builder's mutating method to return this.type, which means it doesn't quality as an unconsumed value. skipField from Java API is ascribed : Unit to fix the inferred type of the match (which was Any or AnyVal) and also indicate that discarding the value was intentional (though Java results don't qualify as unconsumed).

Scala 3 doesn't like using a companion as a function unless it extends function. It is also safe to be explicit in Scala 2.

Two lints are turned off in test because of ScalaTest style.

MiMA exclusions are added for the this.type change, but that could be handled also as : Unit @nowarn at use site.

[som-snytt]

Usually, I wait for test feedback before undrafting. Especially because I don't entirely know what I'm doing here.

[thesamet]

Hi @som-snytt , checking in if you'd like to continue working on this PR?

[som-snytt]

Thanks, I've been reducing my personal backlog and this was still on it. I refreshed my unreliable memory and improved the tweaks. I think I didn't get the warnings right on 2.12, so that will require a fix. I can squash when it looks OK. (Edit: Needed to regenerate files.)