feat(diff): add list-modified-envs flag to tk diff command

[dsotirakis]

Adds a new --list-modified-envs flag to the tk diff command that efficiently identifies and lists only the environment names that have changes, without generating the full diff output.

When working with large Tanka repositories containing many environments, users often need to quickly identify which environments have changes before deciding whether to generate full diffs or proceed with deployments.

After touching ksonnet/environments/my-deployment/main.jsonnet for dev, but this deployment is deployed in all clusters, we can now do:

$ tk diff . --list-modified-envs
environments/my-deployment/dev.my-deployment