Improved version for 2021 rileyjshaw/palette-test-tool

I recently added a color channel splitter back into my site header, so I
dusted off this project. I’m finally settling on a standard palette. This
was further motivated by adding a dark theme to my site.

Some improvements since 2018:

  • Moved this out of the rileyjshaw.github.io repo into its own project
  • Improved UX around color-editing to something much quicker
  • Updated UI to show all colors and blend variants on screen at once
  • Added a toggle to show / hide secondary colors
  • Added saving / export tools
  • Added a toggle for dark mode, and separate state for the two palettes
  • Added a toggle to test different blend modes on the fly
  • Added a button to test live on rileyjshaw.com
  • Simplified code structure, added some dependencies, broke files out
  • Improved tooling