lint(track_config): add initial concepts checks

[ee7]

This commit fully implements these rules:

• The "concepts" key is required
• The "concepts" value must be an array
• The "concepts[].uuid" key is required
• The "concepts[].slug" key is required
• The "concepts[].name" key is required

And partially implements these rules:

• The "concepts[].uuid" value must be a unique, lowercased v4 UUID string
• The "concepts[].slug" value must be a non-empty, non-blank, lowercased string using kebab-case
• The "concepts[].name" value must be a non-empty, non-blank, titleized string

These rules are not yet implemented:

• The "concepts" value must have a entry with a matching "slug" property for each concept listed in a concept exercise's "concepts" property
• Each "concepts" value must have a concept/<concepts.slug>/about.md file.
• Each "concepts" value must have a concept/<concepts.slug>/introduction.md file.
• Each "concepts" value must have a concept/<concepts.slug>/links.json file.

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This PR causes the below diff to the output of configlet lint

research_experiment_1

+Missing key: 'concepts':
+./config.json
+

That is:

• all of the real tracks have a concepts that is at least the empty array.
• all of the tracks that have a non-empty array pass the checks.