Add font-size utilities based on Pajamas typescale specifications

What does this MR do?

Adds .gl-font-size-XX utility classes for setting font-size based on the sizes specified in Pajamas: https://design.gitlab.com/product-foundations/typography#typescale.

I'm open to renaming these classes if there's a better convention to follow.

Why?

I'd like to use one of these utility classes to change the font size of the card headers on the Releases page as part of #31289 (closed).