Add SepReaderHeader.TryIndexOf #448

Job	Run time
pack	1m 17s
format (ubuntu-latest)	38s
test-parsers (ubuntu-latest, Debug)	4m 24s
build (ubuntu-latest, Debug)	1m 14s
test-parsers (ubuntu-latest, Release)	4m 32s
build (ubuntu-latest, Release)	57s
test-parsers (windows-latest, Debug)	6m 31s
build (windows-latest, Debug)	3m 33s
test-parsers (windows-latest, Release)	5m 42s
build (windows-latest, Release)	2m 1s
test-parsers (macos-latest, Debug)	2m 43s
build (macos-latest, Debug)	59s
test-parsers (macos-latest, Release)	2m 46s
build (macos-latest, Release)	53s
create-release-push	0s
	38m 10s

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