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