Add theme.anchor.icon.color

[taysea]

Expected Behavior

Should be able to define an icon color to be used by default in Anchors. I'd expect the logic to be:

1. If the caller defines a color directly as prop on the icon, this color should apply.
2. If the caller defines a color on the Anchor itself, this color should apply to the anchor text + icon (this is current behavior)
3. If no color prop is used on Anchor or icon, then theme.anchor.icon.color should be applied to the icon.
4. If theme.anchor.icon.color is not defined, then just apply the anchor color to icon (this is current behavior).

This will allow easier enforcement across applications of what currently is just guidance: https://github.com/grommet/grommet-theme-hpe/wiki/Migrating-from-HPE-Theme-4.x-to-5.x#updated-anchor-treatment

Actual Behavior

No ability to theme icon color.

URL, screen shot, or Codepen exhibiting the issue

Steps to Reproduce

Your Environment

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