feat(astro): support lessons without parts or chapters (#374) #709

Job	Run time
CLI Integration Tests	1s
CLI Integration Test Matrix (ubuntu-latest, 18)	2m 53s
CLI Integration Test Matrix (ubuntu-latest, 20)	2m 41s
CLI Integration Test Matrix (ubuntu-latest, 22)	2m 40s
CLI Integration Test Matrix (macos-latest, 20)	2m 40s
CLI Integration Test Matrix (windows-latest, 20.13.1)	6m 49s
	17m 44s

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