Add rule to check for usage of NPAPI-based plugins

[alrra]

• [ ] Check if HTML documents use any NPAPI-based plugins.

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See also:

• https://blogs.windows.com/msedgedev/2016/12/14/edge-flash-click-run/
• https://blog.mozilla.org/futurereleases/2016/07/20/reducing-adobe-flash-usage-in-firefox/
• https://blog.google/products/chrome/flash-and-chrome/
• https://webkit.org/blog/6589/next-steps-for-legacy-plug-ins/
• https://www.fxsitecompat.com/en-CA/docs/2016/plug-in-support-has-been-dropped-other-than-flash/
• https://developer.mozilla.org/en-US/Add-ons/Plugins
• https://developer.chrome.com/extensions/npapi

[alrra]

https://blogs.windows.com/msedgedev/2017/07/25/flash-on-windows-timeline/