Move `DashBase.to_dash` type-piracy definitions to DashBase repo

[etpinard]

Dash.jl/src/plotly_base.jl

Lines 1 to 8 in d01ce73

	import PlotlyBase
	import JSON
	function DashBase.to_dash(p::PlotlyBase.Plot)
	data = JSON.lower(p)
	pop!(data, :config, nothing)
	return data
	end

Dash.jl/src/plotly_js.jl

Lines 1 to 7 in d01ce73

	import JSON
	function DashBase.to_dash(p::PlotlyJS.SyncPlot)
	data = JSON.lower(p.plot)
	pop!(data, :config, nothing)
	return data
	end

are defining DashBase.to_dash methods on type the Dash module does not own. This is type-piracy!

We should move these method definitions to https://github.com/plotly/DashBase.jl inside package extension modules with PlotlyBase.jl and PlotlyJS.jl as weakdeps.

[etpinard]

related: #214

[etpinard]

After resolving this issue, we'll be able to drop PlotlyBase as a dep.

Moreover, we'll be pretty close to dropping all references to the JSON package, with the exception of

Dash.jl/gen_resources/generator/github.jl

Line 45 in d01ce73

reply = JSON.parse(HTTP.payload(resp, String))

Dash.jl/test/integration/dash_assets/test_dash_assets.py

Line 27 in d01ce73

tested = json.loads(dashjl.wait_for_element("#tested").text)