Add methods Reader.getSource and Reader.getSourceLines #2283

Job	Run time
Build (11, temurin, ubuntu-latest)	9m 17s
Build on Windows (11, temurin, windows-latest)	12m 12s
Build (11, temurin, macos-latest)	13m 12s
Build on Windows (17, temurin, windows-latest)	12m 12s
Build (17, temurin, ubuntu-latest)	8m 48s
Build on Windows (21, temurin, windows-latest)	12m 17s
Build (17, temurin, macos-latest)	15m 4s
Build (21, temurin, ubuntu-latest)	9m 1s
Build (21, temurin, macos-latest)	11m 38s
Test Asciidoctor Upstream (11, ubuntu-latest, 3.9.4)	8m 36s
Test Asciidoctor Upstream (17, ubuntu-latest, 3.9.4)	8m 53s
Test Asciidoctor Upstream (21, ubuntu-latest, 3.9.4)	9m 27s
	2h 10m 37s

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