async heading-anchors, prevent FOAT, housekeeping

[rdela]

• 8100561 color transparent and font-size 0 for main img
  prevents loading FOAT! Flash of alt text
• 6e319e2 async heading-anchors.js

  As with regular scripts, async causes the script to download without
  blocking the HTML parser and executes as soon as possible. Unlike
  regular scripts, async also works on inline modules.
  https://jakearchibald.com/2017/es-modules-in-browsers/#async-works-on-external--inline-modules

• 213c75a clean up config, combine imports
• 75bd941 alpha to beta, edit for brevity

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