move to ESM

[LitoMore]

Closes #256
Closes #258

Changes

• Require Node.js 18
• Move to ESM
• Add type definitions
  • Most of them are from the latest @types/color, but I made some tweaks
• Update GitHub Actions settings
• Bump color-convert@3
• Bump color-string@2

@Qix- I didn't list all converter use cases in index.test-d.ts. Please let me know if you want to them all.

And some tests are missing, like .toJSON(). Should I fix them in this PR?

[Qix-]

I didn't list all converter use cases in index.test-d.ts. Please let me know if you want to them all.

I'd just test a few random, weird ones, but doing all of them isn't necessary IMO.

And some tests are missing, like .toJSON(). Should I fix them in this PR?

If you'd like. I think that functionality is pretty standard, iirc.

[LitoMore]

@Qix- .toJSON() tests added now.

[Qix-]

Looks good, merge when ready :)

[Qix-]

Published as 5.0.0 :) thanks!