SepWriter: Add async support (C# 13+ only) #731

Job	Run time
format (ubuntu-latest)	53s
pack	3m 23s
build (ubuntu-latest, Debug)	1m 59s
test-parsers (ubuntu-latest)	3m 48s
build (ubuntu-latest, Release)	1m 5s
test-parsers (windows-latest)	7m 31s
build (windows-latest, Debug)	3m 37s
test-parsers (macos-latest)	3m 23s
build (windows-latest, Release)	3m 15s
build (macos-latest, Debug)	1m 28s
build (macos-latest, Release)	1m 20s
create-release-push	0s
	31m 42s

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