Add textColor prop to <Button /> to allow overriding of colors

[pfarejowicz]

This change is inspired by the limitations of the gestalt Button with different colored backgrounds. Previously, the white button worked well against a darkGray background as it matched in color.

However, with user education now using the Pinterest blue and error states using the Pinterest red, this no longer scales to those backgrounds

With this change, the user of Button can pass an optional textColor prop to override that font color and make the text color match against the background of the container parent.

This matches 1:1 with what the PDS spec says user education and error states is supposed to be, but we previously never followed this guideline.

Before this change, developers would have to hack together custom buttons to get blue text but with this new prop, it will be included in gestalt and we can remove those hacks from the codebase.

[btraut]

How restrictive do you think we should be? This change allows users to create red buttons with red text (and a few other illegal states). Instead, we could just create color schemes that encompass both button and text color. I'm not saying at all that this is the right approach, but I wanted to make sure you considered it.

[pfarejowicz]

How restrictive do you think we should be? This change allows users to create red buttons with red text (and a few other illegal states). Instead, we could just create color schemes that encompass both button and text color. I'm not saying at all that this is the right approach, but I wanted to make sure you considered it.

Yeah I considered that possibility of abuse, my mindset was designers and developers here at Pinterest are not going to go off-spec and create color combinations like that so I gave them the trust to control their own button colors and text colors with the hope they only use it for special cases like white - while leaving the option for the open source world to do whatever. The alternative would probably be extending color from what it is now blue | red | white | ... to add white-red and white-blue to the list so its more limiting. Is that something you'd advise, I'm open to both solutions?