Identify compiler used to build plugin

We could try and identify what source / compiler was used to compile the plugin:

- plain old Java
- Kotlin
- dynamic Groovy
- static Groovy

The issue proposes identifying whether a plugin was built with plain Java, Kotlin, dynamic Groovy, or static Groovy. No files, tests, or entry points are named, so first inspect the analyzer's existing bytecode-analysis flow and determine what evidence it exposes. Done means there is a defined and testable way to distinguish the listed cases.

Written by the indexing model from the issue text.