Add Danger checks for feature flag YAML definition files

What does this MR do and why?

This adds Danger checks and diff suggestions for feature flag definition files, depending on the FF type.

I also enabled the Danger check locally as the feature flag definition file can actually be checked locally.

Screenshot 1	Screenshot 2	Screenshot 3

How to set up and validate locally

You can create a FF definition file and run the pre-push Lefthook hook:

lefthook run pre-push

Related to #428751.