Add ability to add output prefixes to the REPL output and use that to implement an IPython mode

[KristofferC]

With

atreplinit() do repl
    if !isdefined(repl, :interface)
        repl.interface = REPL.setup_interface(repl)
    end
    REPL.ipython_mode(repl)
end

in the startup.jl file:

Only the first commit is needed for a package to implement the IPython mode (except prompt pasting which will not work from a package and currently requires a source change in REPL to modify the searched prefix).

Fixes #46310.

cc @eschnett

[KristofferC]

Seems pretty harmless and fun so I'll merge.

[hyrodium]

Is it correct to call this feature "ipython"? I'm understanding the origin of the format like In[1] and Out[1] is Mathematica.

• The first version of ipython was released in 2001 (https://en.wikipedia.org/wiki/IPython).
• The Out function in Mathematica was introduced in 1998.

[svretina]

since the name changed, if anyone comes looking how to enable the numbered promt in startup, here is the snippet with the updated name:

atreplinit() do repl
    try
        @eval using REPL
    catch e
        @warn "error while imporing REPL" e
    end
    if !isdefined(repl, :interface)
        repl.interface = REPL.setup_interface(repl)
    end
    REPL.numbered_prompt!(repl)
end