Repl line continuation prompt prefix

[roberth]

Is your feature request related to a problem?

nix-repl> ((((foo) bar) baz)

Is it running or is it waiting for input?
Kind of hard to tell, because if it were waiting for input, nix repl doesn't provide a prompt line (except for some indentation, which isn't nearly as obvious).

Proposed solution

nix-repl> (((foo bar baz))
        >

(as it was waiting for input)

Alternative solutions

A spinner and/or progress bar when evaluating; slightly trickier maybe, because printValue relies on disabling the progress bar.

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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