Multiline continuation other than unmatched brackets

[MartinNowak]

Currently when one of these brackets '[]', '()', '{}' is unmatched the REPL will go into multiline mode. Multiline input can be aborted with two blank lines.
Additionally to the unmatched brackets there should be an explicit way to break input lines.

@callumentator proposed to use Shift+Enter for adding a new non-breaking line.

Alternatively escaping the newline with \ would make sense.

iota(0, 20)\
.map!(a => 2 * a)