Fix layout of REPL completions with newlines

[Liozou]

Fix #45836 by forcing REPL completions that include a newline to print on a single column.

The behaviour on this PR is like for a small terminal:

julia> include(<tab>
include(mapexpr::Function, fname::AbstractString)
     @ Base.MainInclude client.jl:469
include(fname::AbstractString)
     @ Base.MainInclude client.jl:470

I didn't know how to test this so feel free to comment if you think the test can be improved.

[Liozou]

CI failure looks like #45310 and #45981, which should be unrelated.

[KristofferC]

I think this is fixed by #46241, shout otherwise :)

[vtjnash]

I think this is still merge-worthy. Not specifically needed for that case any more, but any other random user might come along and use the REPLCompletions code and expect this case to work.