repl: Customizable subprompt for multiline input #1346

Job	Run time
Build slim tarball	1m 19s
aarch64-linux: with shared libraries	2m 55s
x86_64-linux: with shared libraries	2m 59s
aarch64-darwin: with shared libraries	3m 1s
x86_64-darwin: with shared libraries	3m 0s
	13m 14s

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