feat: Add ability to clear color span range

[Sermilion]

1. We had a case where we needed to remove color span instead of toggling. Setting Color.Unspecified did not work. I had to pass in exact Span with color to clear. A cleaner way could be to pass in Color.Unspecified, which would just remove current color span. First commit addresses that case.

2. When text has multiple color spans and I want to select whole text and remove all color spans - it is currently impossible. Second commit added ability to clear all color spans for current selection.

I cant add reviewers, so tagging you directly @MohamedRejeb