feat: allow other entrypoints

[Duologic]

The change allows the user to run tanka with alternative entrypoints, not limiting to main.jsonnet anymore.

This opens the door to using tk eval for purposes outside of the Tanka/K8s Environments paradigm. For example, one can
write a library with a test.jsonnet and run tk eval test.jsonnet in that library. Caveat: it needs to have a
tkrc.yaml or jsonnetfile.json so Tanka can resolve the JPATH. It also works for tk export and tk show, beware
that it will add default namespace to kubernetes objects, even though a spec.json might not exist.

This change is backwards compatible and still defaults to main.jsonnet if no file is given, tk diff will still want a
spec.json to connect to a cluster.

Feel free to give feedback/pushback on the idea here too.

I've done a grep on main.jsonnet in the codebase to set this up, but there are many other valid references to
main.jsonnet left that don't need replacing.

[sh0rez]

Adding lifecycle/blocked here because Tanka is currently in feature-freeze (not accepting features not required for the release), so we can get 0.12 out.

Will review once released

[Duologic]

rebased

[Duologic]

rebased.

[sh0rez]

LGTM!! Very awesome work!