Define JSON schema for score spec

[Susa Tünker]

Define JSON schema for the score spec to make sure that users are aware of which properties are supported and whether these are optional or required. This could be put into practice via a VS Code extension, a lint command line flag or similar.
Note: Should be stored in docs repo.

[Susa Tünker]

This issue is comprised of 3 subtasks:

• Define JSON schema for score spec

• Incorporate the schema into the Score parsing library to ensure that each Score file is validated

• Automatically generate docs from the schema