Make FileSystemWriter & Writer classes abstract. #112

Job	Run time
test-typescript-sdk-r4 (latest)	13s
test-mustache-java-r4-example (latest, 21)	1m 13s
test-csharp-sdk (latest, 8.0.x)	1m 16s
test-mustache-java-r4-example (latest, 25)	1m 17s
test-typescript-ccda-example (latest)	25s
	4m 24s

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