Allow for pattern matching for directories in WEB_FEATURES.yml

[jcscottiii]

Currently, developers can only specify files, file patterns or the ** wildcard.

It would be nice to allow developers to specify directories and directory patterns as well.

For example: In webaudio, it would be nice to do this.

features:
- name: web-audio
  files:
  - "**"
  - "!the-audio-api/the-audioworklet-interface*"
  - "!the-audio-api/the-offlineaudiocontext-interface*"

But currently, a developer would need to create YAML files in all the other subdirectories of webaudio with these contents:

features:
- name: web-audio
  files:
  - "**"

On a semi related note: It would be nice to alert users that their new test will apply to a feature due to an existing YAML definition.