feat(cli): support cdktn.json config file

[gabegorelick]

Support cdktn.json in addition to cdktf.json. When both are present, cdktn.json will be used.

See #152

Related issue

Fixes #152

Description

Extra config file resolution into shared utilities. Those utilities check for cdktn.json and fallback to cdktf.json if cdktn.json does not exist.

cdktf.json is still referenced in many of the examples and docs, and it is still emitted by cdktn init. Followup work can update those instances.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes