Use the same plotlyjs version number for all imports

[fonsp]

This PR makes sure that the same version of the plotlyjs web asset is used for all imports: through artifact or through CDN.

This also adds _js_version and _js_path as public API: this means other packages can depend on PlotlyJS.jl to get the plotly.min.js asset. Used in JuliaPlots/Plots.jl#4862

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (43d38e9) 37.50% compared to head (b2532bc) 36.51%.
Report is 5 commits behind head on master.

Files	Patch %	Lines
src/PlotlyJS.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   37.50%   36.51%   -0.99%     
==========================================
  Files           4        4              
  Lines         288      293       +5     
==========================================
- Hits          108      107       -1     
- Misses        180      186       +6     

Flag	Coverage Δ
unittests	36.51% <50.00%> (-0.99%)	⬇️

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[fonsp]

Thanks! Could you also make a release? Then we can work on JuliaPlots/Plots.jl#4862

[fonsp]

Thanks again :)