Use data URL for local plotly asset

[fonsp]

Currently, when including the plotly.min.js locally in HTML, this is done using a file:// URL. Importing scripts using a file URL is only allowed in browsers if the page origin is also a file URL, i.e. if you open file://...something.html.

This PR uses Base64 to include the script using a data URL, which is supported in all environments, including http and https origins. This means that the plotly backend can be used with _use_local_dependencies[] in web-based IDEs like Pluto and other web sites.

Fix #4854, depends on #4862

[pankgeorg]

This PR doesn't depend on #4862

I see #4884 as a better alternative to #4862 (which would defeat the purpose of having the plotly() backend, but the artifact there is 110MB, which is.. not ideal for a package that already takes a long time to ]add).

So my suggestion is, can we move forward with this PR (which unblocks the issue with file:/// URLS over insecure contexts, until we figure out a nice way to depend on the artifacts? (I also don't want to plagiarize this code in my other PR 😅)

@isentropic, @fonsp proactively thank you both 🙏🏽

[codecov]

Codecov Report

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

Comparison is base (7cd27c4) 90.29% compared to head (32468f9) 89.77%.
Report is 30 commits behind head on master.

Files	Patch %	Lines
src/init.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4863      +/-   ##
==========================================
- Coverage   90.29%   89.77%   -0.53%     
==========================================
  Files          40       40              
  Lines        8752     8780      +28     
==========================================
- Hits         7903     7882      -21     
- Misses        849      898      +49     

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[BeastyBlacksmith]

Looks good to me and thanks for taking care 🙂