Fix Markdown print error in 1.12.0-DEV

[huangyxi]

Fix #357 corresponding to JuliaLang/julia#51928
where Markdown.lines has been removed:

https://github.com/JuliaLang/julia/blob/3a35aec36d13c3e651c97bac664da2e778d591ad/stdlib/Markdown/src/render/terminal/formatting.jl#L10

[huangyxi]

Performance improvement:

if isdefined(Markdown, :lines)
    split_lines(s::AbstractString) = Markdown.lines(s)
else
    split_lines(s::AbstractString) = split(s, '\n')
end

[KristofferC]

Don't think it will matter