Allowing euiPaletteColorBlind() function to have both light AND dark alternates

[cchaos]

While trying to update the Visualizations in Kibana to use the new palette service. I realized that the alternates provided by euiPaletteColorBlind() only went one direction (lighter). We should also provide a way to make darker variations so that we can swap this legend color picker with our color palette.

So now, euiPaletteColorBlind()' s third parameter direction accepts 'lighter' | 'darker' | 'both'. The docs example has been updated to:

And I've added some props documentation for the function.

Checklist

• Check against all themes for compatibility in both light and dark modes
• [ ] Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• [ ] Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[thompsongl]

Code changes LGTM

[elizabetdev]

Tested locally and checked the code and LGTM! 🎉

Just a suggestion, maybe you could somewhere in the text specify what are the arguments that we can pass to the function.

[cchaos]

@miukimiu I did manually add the function and it's props(parameters) to the Props tab of the sections.