Rewrite Javadoc integration, update to minimum JDK 9

The current Javadoc tool was deprecated in JDK 9, and removed entirely in JDK 13. Thus, Kodiak's Javadoc integration only runs in older JDKs < 13.

Kodiak should be updated to use the new JDK Javadoc structure.

Like the new Dokka integration, it would be nice to distribute this integration as a standalone Javadoc plugin, which can be configured to run on its own directly within Gradle for proper dependency/type information.

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

Start by reading the javadoc-formatter project and compare its approach with the Dokka integration. Then inspect javadoc-runner for the JDK Javadoc CLI changes; javadoc-models is expected to remain unchanged. Done means the formatter supports the new JDK Javadoc structure, the runner works with the new CLI, and the integration can be distributed as a standalone Gradle plugin.

Written by the indexing model from the issue text.