feat: add electronegativity plugin

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).

Summarize your changes:

Adds a first-party plugin for Electronegativity, which runs after packaging is done.

There is an associated PR for the electronforge.io docs.

[codecov]

Codecov Report

Merging #1900 into master will not change coverage.
The diff coverage is n/a.

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##           master    #1900   +/-   ##
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  Coverage   72.71%   72.71%           
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  Files          74       74           
  Lines        2210     2210           
  Branches      416      416           
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  Hits         1607     1607           
  Misses        443      443           
  Partials      160      160           

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[malept]

It might be interesting at some point to parse the output of the Electronegativity API call and throw an error if, for example, there are high severity items found. But this is a good first pass.