Update concept slugs to add "What are builtin commands?" concept

[TropicolX]

Added 'what-are-builtin-commands' to concept slugs for various stages.

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Note

Adds the what-are-builtin-commands concept to the course and links it to the exit, echo, and type stages.

• Course config (course-definition.yml)
  • Add what-are-builtin-commands to top-level concept_slugs.
  • Tag stages with concept_slugs:
    • pn5 (Implement exit) → what-are-builtin-commands
    • iz3 (Implement echo) → what-are-builtin-commands
    • ez5 (Implement type) → what-are-builtin-commands

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