Rewrite Dokka integration as a proper, standalone Dokka Plugin

In Kotlin 1.4, Dokka was completely rewritten with a proper mechanism for implementing plugins. To keep up, Kodiak needs to have its Dokka integration rewritten with the new Dokka plugin architecture.

Specifically, The `dokka-formatter` project should be rewritten and published as a proper Dokka plugin. It should be able to be used in any other project looking to generate JSON as outputs, rather than HTML, Markdown, etc. But, it is also able to be embedded/run within the `dokka-runner` project as normal.

It would also be nice to include an option for the `dokka-runner` to look for sources already generated by the normal Dokka Gradle tasks, which were generated using this formatted. That way, we get all the benefits of proper dependency/type information, and it would also run faster (but carries the implication that Dokka must be configured/run separately from Orchid first).

Note that this should really only be a rewrite of the `dokka-formatter` project, and minor updates to `dokka-runner` to work with the new CLI. `dokka-models` likely should not need any changes.

https://kotlin.github.io/dokka/1.4.32/developer_guide/introduction/