New feature for ::JuliaProgram: `@extractvars` used in `p.main(inputs, outputs)`: generate vars from inputs and outputs

[cihga39871]

# assume p isa JuliaProgram and has inputs A, B, and outputs C.
p.main(inputs::Dict{String}, outputs::Dict{String}) = begin
    A + B + C
end

Check whether we can make this happen if wrapping p.main in a function like this:

function outer_main(inputs::Dict{String}, outputs::Dict{String})
    @extractvars
    p.main(inputs, outputs)
end

[cihga39871]

See v0.8.
Use quote instead