Make Classpath Configurable with Env Variable

[tricktron]

Frege modules with dependencies to other jars cannot be resolved:

A low hanging fruit solution:

Expose a FREGE_LSP_EXTRA_CLASSPATH variable to manually add third party dependency jars.

Nice user experience solution:

Use the frege gradle configuration value. This means a lot more effort:

• Add a custom model to the frege gradle plugin
• Add the gradle tooling api to this repo and integrate it with the custom model.

See https://stackoverflow.com/questions/34008335/gradle-tooling-api-get-task-outputs