Moshi code gen should warn about referencing an Enum that does not have `@JsonClass` to avoid R8 issues

[yogurtearl]

When using R8 we want to keep everything that is used by code gen'd adapters, including all the value of enums referenced.

This works fine if you put @JsonClass on all the enums used in JSON, but it is easy to miss that.

I propose that Moshi code gen issue a warning or and error if any @JsonClass references an enum that is not marked with @JsonClass

[ZacSweers]

We use lint checks for this at slack, which are a little more configurable. We open sourced our Moshi lints here: GitHub.com/slackhq/slack-lints