Improve show for nested Objects

[joshday]

This PR improves the usefulness and performance of Base.show-ing nested JSON3.Objects.

Here's an example with the Plotly.JS schema. It takes several seconds to show the Object, and no information about the nested objects has been provided:

path = download("https://github.com/plotly/plotly.js/raw/v2.35.2/dist/plot-schema.json");

obj = JSON3.read(read(path));

@time display(obj) # warmup
@time display(obj)
# JSON3.Object{Vector{UInt8}, Vector{UInt64}} with 7 entries:
#  :animation  => {…
#  :config     => {…
#  :defs       => {…
#  :frames     => {…
#  :layout     => {…
#  :traces     => {…
#  :transforms => {…
#  3.476441 seconds (157.89 M allocations: 8.390 GiB, 11.97% gc time)

With this PR:

@time display(obj)
#JSON3.Object{Vector{UInt8}, Vector{UInt64}} with 7 entries:
#  :animation  => { direction, frame, fromcurrent, mode, transition }
#  :config     => { autosizable, displaylogo, displayModeBar, doubleClick, doubleClickDelay…
#  :defs       => { editType, impliedEdits, metaKeys, valObjects }
#  :frames     => { items, role }
#  :layout     => { layoutAttributes }
#  :traces     => { bar, barpolar, box, candlestick, carpet, choropleth, choroplethmap, cho…
#  :transforms => { aggregate, filter, groupby, sort }
#  0.003864 seconds (18.37 k allocations: 1.214 MiB)

---

Note that the top level show method uses show(::IO, ::MIME"text/plain", ::AbstractDict) and nested objects use show(::IO, ::Object).

This will break any downstream code that relied on Base.show(::IO, ::Object) to generated valid JSON. If valid JSON is a requirement I'd propose changing Base.show(io::IO, obj::Object) = JSON3.write(io, obj). It's still slow at about 1 second, but it at least would show some of the contents:

julia> @time display(obj)
#JSON3.Object{Vector{UInt8}, Vector{UInt64}} with 7 entries:
#  :animation  => {"direction":{"description":"The direction in which to play the frames tr…
#  :config     => {"autosizable":{"description":"Determines whether the graphs are plotted …
#  :defs       => {"editType":{"layout":{"description":"layout attributes should include an…
#  :frames     => {"items":{"frames_entry":{"baseframe":{"description":"The name of the fra…
#  :layout     => {"layoutAttributes":{"_deprecated":{"title":{"description":"Value of `tit…
#  :traces     => {"bar":{"animatable":true,"attributes":{"_deprecated":{"bardir":{"descrip…
#  :transforms => {"aggregate":{"attributes":{"aggregations":{"items":{"aggregation":{"edit…
#  0.894440 seconds (38.21 M allocations: 1.984 GiB, 9.98% gc time)

[joshday]

Failing CI looks unrelated to this PR (Some Int64 vs Int32 error for Arrow compatibility on Windows, invalid kwarg to makedocs in the Documenter build)

[joshday]

I'm opting for Base.show(io::IO, o::Object) = JSON3.write(io, o)

It seems the least disruptive and still a win.