Add support for YAML

[dblock]

Check for existing issues

• Completed

Describe the feature

Coming from opensearch-project/opensearch-api-specification#599 and an attempt to use Vale in opensearch-project/opensearch-api-specification#620, we'd like to use Vale as a style guide for content in YAML files. Our YAML files are source code for an OpenAPI spec, so we would like to be able to say things like "only check the field called description".

(Could attempt to contribute with some pointers of where to start.)

[kolchfa-aws]

@dblock Seems that there is a similar closed issue, which contains a link to the blog with implementation options

[jdkato]

I think such processing of data-serialization formats is out of scope for Vale. #383 is another related issue.