Add "skills" (plural) as accepted key to manifest.yml

[krisgesling]

Currently Skill authors may define a list of other Mycroft Skills required using a skill list in their manifest.yml.

Given the ease and likelihood of people using the plural form "skills", I wonder if we just add it as an accepted block. This may also makes more sense for users given it is a list of one or more Skills.

Relevant documentation:
https://mycroft-ai.gitbook.io/docs/skill-development/skill-structure/dependencies/manifest-yml

Relevant code:
msm/skill_entry.py