feat: add bash-like linebreak (\ at the end of line) to the REPL

[khlopkov]

Hello, this PR was initially submitted to upstream repository here

Current REPL doesn't support multiline input. Found few issues marked as enhancement:

• Add ability to create multiline input in REPL influxdata/flux#458
• Improve REPL interface influxdata/flux#50

This PR suggests to interpret \ at the end of the line as a limiter for a new line like in bash.

Checklist

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[lmangani]

Seems very useful and I do not see any negative impact - any comments from the Influx team?