Parse config files as YAML

[sbliven]

Since YAML is a strict superset of JSON, we can accept YAML for all config files in a fully backwards-compatible manner. This is also fully supported by NestJS.

1. Add js-yaml and @types/js-yaml packages
2. Replace json.load with yaml.load
3. Accept .json, .yaml, and .yml file extensions for the config files (stopping with the first one found)

[bpedersen2]

Good idea! 👍

[despadam]

As seen also in the comment here: #841 (comment)