support JSON v1

[hhaensel]

After the latest JSON v1 has come out, I think it would be a cool for WebIO to support that version.
I've just adapted an older PR to AssetRegistry to also cover JSON v1 compatibility and hope that it will be merged. So the main task would be to replace the show_json() method and its call in syntax.jl. As I am not a frequent user of WebIO, I'm perhaps not the best person to do this.
Just as a motivation, I think PlotlyJS and all other comsumers of WebIO would be extremely happy if we could leave the old JSON/JSON3 schism behind.

[hhaensel]

Just had a look into syntax.jl and found some strange definitions:

struct Interpolator{S<:AbstractString}
    s::S
end

macro js_str(s)
    writes = map(Interpolator(s)) do x
        if isa(x, AbstractString)
            # If x is a string, it was specified in the js"..." literal so let it
            # through as-is.
            :(write(io, $(x)))
        else
            # Otherwise, it's some kind of interpolation so we need to generate a
            # JavaScript representation of whatever it is/whatever it evaluates to.
            :(showjs(io, tojs($(esc(x)))))
       end
   end

   :(JSString(sprint(io->(begin; $(writes...) end))))
end

As Interpolator(s) expects an an s to be an AbstractString , @js_str([1, 2, 3]) will throw an error. So we will never reach the else block of isa(x, AbstractString). But that's the only occurence of showjs(). So do we need showjs() at all?
I would have a v1 compatible version that can live without any JSEvalSerialization.

function showjs(io, x::Any; pretty = verbose_json[] && !isa(x, AbstractString), kwargs...)
    JSON.json(io, x; pretty, kwargs...)
end

Moreover, I am a bit puzzled by

"""
    tojs(x)

Returns a JSString object that constructs the same object as `x`
"""
tojs(x) = x

Shouldn't that be something like

tojs(x) = JSString(json(x))
tojs(x::AbstractString) = JSString(x)

Happy to file a PR if I get some comments. I'd proposed to make a version that supports both JSON <v1 and v1.

EDIT: updated proposal for tojs()

[hhaensel]

Update: I could get WebIO precompile with JSON v1 with lots of other packages requiring updates as well, some only for testing:

• Conda
• AssetRegistry (Add Project.toml, speedup of isregistered and getkey, path consistency AssetRegistry.jl#14)
• JSExpr (enhance compatibility with JSON v1 JSExpr.jl#48)
• Blink (testing)
• IJulia (testing)

I also tried to get PlotlyJS running (for which I am collaborator) and mostly succeeded, however, I saw a difficulty converting between JSExpr.JSString and WebIO.JSString. They are defined identically and there's lot of code duplication.

Is there any reason for WebIO not to import JSExpr.JSString and JSExpr.tojs()?

EDIT: added PRs and tojs() in the last line
EDIT 2: Sorry, I didn't realise that JSExpr is not loaded by default, just part of the test environment, so please ignore the comment above, maybe I have to fix that inside PlotlyJS then