feat(library/script_evaluator): add abstract class that exposes only the API needed by frontend objects

The main motivation is to break the remove the dependency frontends/lean <-- bindings/lua.
This dependency is undesirable because we want to expose the frontends/lean parser and pretty printer objects at bindings/lua.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/script_evaluator): add abstract class that exposes only the API needed by frontend objects