method TAB completion doesn't work properly with multiline input

[staticfloat]

When attempting to TAB-complete to get method prompting with a multi-line input, the results seem to get confused:

julia> println(

prompts:

println(io::IO) @ Base coreio.jl:5              println(io::IO, xs...) @ Base strings/io.jl:75
println(xs...) @ Base coreio.jl:4

However:

julia> x = 2
       println(

Gives all of Base.

We should improve our TAB-completion to no longer do this, as it's quite useful when writing multi-line commands.

[Keno]

We currently rely too much on ad hoc string processing in the completion code. This kind of stuff would be much easier with a proper error-recovering parser like we have in JuliaSyntax.jl, so we should focus our efforts on getting that integrated, so we can use it in places like this.