Make indentation configurable and default to d2 fmt indent #41

Job	Run time
build (ubuntu-latest, 26.3)	29s
build (ubuntu-latest, 27.2)	30s
build (ubuntu-latest, 28.2)	33s
build (ubuntu-latest, snapshot)	35s
build (macos-latest, 26.3)	56s
build (macos-latest, 27.2)	51s
build (macos-latest, 28.2)	1m 40s
build (macos-latest, snapshot)	1m 24s
build (windows-latest, 26.3)	3m 11s
build (windows-latest, 27.2)	1m 26s
build (windows-latest, 28.2)	1m 1s
build (windows-latest, snapshot)	2m 47s
	15m 23s

Provide feedback