repl: Customizable subprompt for multiline input #1348

Job	Run time
Build slim tarball	1m 29s
aarch64-darwin: with shared libraries	25m 25s
x86_64-linux: with shared libraries	4h 32m 8s
aarch64-linux: with shared libraries	6h 0m 15s
x86_64-darwin: with shared libraries	59m 20s
	11h 58m 37s

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