Add 'moon_phase' to Dark Sky sensor

[fabaff]

Description:

Add 'moon_phase' to Dark Sky sensor.

Related issue (if applicable): fixes 65461

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#6076

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: darksky
    api_key: !secret darksky_api
    monitored_conditions:
      - moon_phase

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io