added text-gray-dark class

[eddyerburgh]

Added a utility class .text-gray-dark. See #19842

[cvrebert]

The premise for why this class would be useful seems reasonable enough.
But this should probably use $body-color instead and perhaps be named something like .text-color-default.
@mdo Thoughts?

[eddyerburgh]

Ok, I'll update the request if @mdo agrees

[RyanZim]

@mdo ping?

[mdo]

If the class is using the specific variable and value, it makes sense to not use the $body-bg. If the classes were .text-body, then maybe, but that's an unclear utility.