Add font-weight utility classes

[tarkhov]

// Text weights - https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight#Common_weight_name_mapping
.text-black {
  font-weight: 900;
}

.text-extra-bold {
  font-weight: 800;
}

.text-bold {
  font-weight: 700;
}

.text-semi-bold {
  font-weight: 600;
}

.text-medium {
  font-weight: 500;
}

.text-normal {
    font-weight: 400;
}

.text-light {
  font-weight: 300;
}

.text-extra-light {
  font-weight: 200;
}

.text-thin {
  font-weight: 100;
}

Pull request #18440

[cvrebert]

CC: @mdo for a decision on the desirability of such classes

[tarkhov]

Thank you.

[mdo]

I don't see a huge need for most of these. The only two I'd consider on the weight side is normal and bold as those apply rather universally to default web fonts.

[mdo]

Added the ones I mentioned in 26da610.