refactor: cleanup colors

[markov00]

Summary

During the heatmap legend fixes I've noticed that we have multiple places where some standard colors are allocated.
I've moved the Color type + some static colors (red, white, black, transparent) into the common/color.ts file.
This is not a complete refactoring but the first part of the initiative to cleanup the folders the typings.

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)

[nickofthyme]

LGTM, I would suggest a slightest simplified API. See comments.

[monfera]

LGTM, good to merge once it passes and conflicts are resolved

Switching to some color object (or fake enum) would be sensible if we expect significant growth to the count of these constants, or a COLOR_ prefix at least. With the current handful of colors, I don't feel strongly either way. Worth doing a last minute grep for all string occurrences 'black' or even without quotes to ferret out template string occurrences or similar

[markov00]

just an update: I've found that transparent and rgba(0,0,0,0) are not treated the same way in the partition chart. I'm currently investigating to fix this

[markov00]

close in favour of #1397