Highlight prompts in REPL syntax highlighting

The julia-console.cson grammar is now in use for syntax highlighting of Julia-REPL code blocks on GitHub:

julia> foo(x) = 2*x
foo (generic function with 1 method)

julia> foo(2)
4

julia> rand(3, 3)
3×3 Matrix{Float64}:
 0.0584225  0.130169  0.71839
 0.374485   0.915134  0.546367
 0.757699   0.195205  0.549311
 
julia> d = Dict(1 => 'a', 2 => 'b')
Dict{Int64, Char} with 2 entries:
  2 => 'b'
  1 => 'a'
 
(jl_jQnjZ8) pkg> st
Status `/private/var/folders/lx/07x6z_b908gd4wd3v_wf2b4c0000gn/T/jl_jQnjZ8/Project.toml` (empty project)

It would be nice to highlight REPL prompts similar to the way they are highlighted in the REPL, e.g. julia> in green, pkg> in blue, help?> in yellow and shell> in red.

Start with the linked grammars/julia-console.cson grammar and inspect how Julia-REPL code blocks are currently tokenized. Compare the prompt forms shown in the issue, then make julia>, pkg>, help?>, and shell> visibly distinct in their requested colors. Done means each listed prompt is highlighted correctly without changing the surrounding REPL output.