fix: force yaml loading to confirm to json-compatible types

[erunion]

This fix was taken from a fix of mine upstream in APIDevTools#247:

While investigating some @apidevtools/swagger-parser quirks I discovered that loading a YAML file that contains a date-like string causes that date to be converted into a Date object, causing the API definition to fail validation.

This updates the usage of js-yaml to ensure that all YAML content conforms to JSON-compatible types by way of its schema option (docs here).