De-JS-ify build tooling

[asaaki]

The gulp stuff is old and dusty, the dep tree has also old and vulnerable stuff in it now.
Also I don't like to constantly check if any dep update could have caused some breaking changes, which alas happens way too often for my taste in the JS ecosystem. And I am not a full time JS dev to spend my time on this.

Also just a lame excuse to build my own tooling in a language of my chosing.

What the toolchain needs to cover (not in that specific order):

• HTML post processing
• HTML clean up (tidy)
• IMG tag post processing
  • add alt attribute
  • add lazy loading if unset
  • add dimensions
  • add LQIP/SQIP (background css)
  • convert to PICTURE tag (w/ webp source + png/jpeg img); leave GIFs alone for now
• minimize HTML
  • note: skip the feeds first!
• minimize JS
• minimize CSS
• image conversions/transformations:
  • optimize/minize source images (mostly PNGs, maybe JPEGs, not sure about GIFs)
  • create WEBPs; note: discard if file size would be bigger than source!
  • create LQIP/SQIP versions (make sure SVG is optimized as well)
  • create thumbnails (mostly for hero images)

These are the basic steps, and most of them come from some JS tools.

If full rewrites are not possible, check alternative tools:

• image post processing: https://github.com/chrisdwheatley/netlify-plugin-image-optim/blob/master/index.js

Some new steps maybe:

• html validation: https://html-validate.org/usage/index.html