Prettify printing of a small number of completions

[JamesWrigley]

Previously show_completions() tended to maximize the number of columns used, and if there were few enough completions to only use one or two lines the text would be a little difficult to read. Now it tries to minimize the number of columns, and there's a minimum number of completions (multicolumn_threshold) needed before it'll start using multiple columns.

Currently multicolumn_threshold is 5, which looked good to me.

Before:

julia> first(1,\t
first(itr, n::Integer) @ Base abstractarray.jl:496  first(x::Number) @ Base number.jl:111

After:

julia> first(1,\t
first(itr, n::Integer) @ Base abstractarray.jl:496
first(x::Number) @ Base number.jl:111

How to test with Infiltrator.jl:

1. Insert Main.@exfiltrate at the beginning of show_completions().
2. Trigger show_completions() with julia> first(\t and remove the call to Main.@exfiltrate.
3. Test with the variables in the safehouse:

   julia> REPL.LineEdit.show_completions(safehouse.s, safehouse.completions[1:5])
   
   first(r::StepRangeLen) @ Base range.jl:833
   first(r::LinRange) @ Base range.jl:834
   first(x::Number) @ Base number.jl:111
   first(S::Base.IdentityUnitRange) @ Base indices.jl:414
   first(iter::Base.SCartesianIndices2{K}) where K @ Base reinterpretarray.jl:259

[JamesWrigley]

Not quite sure what's going on with the failing jobs, the errors don't look related to this change 🤔