Add GT America Standard Medium font face for non-bold usage

[brentswisher]

The problem
Currently, our main san-serif font, GT America Standard is available only in the following weights and variants:

• Weight 400 which serves GT-America-Standard-Regular
• Weight 700 which serves GT-America-Standard-Medium

This makes it so that the only way to use the "Medium" variant is to use the bold (700) weight

Sometimes however, our designers wish to use the medium variant at a non-bold weight, like 500.

The solution
Add a variant for GT America Standard to our fonts.css that uses GT-America-Standard-Medium for the font variant when the weight is set to 500.

Alternatives considered
@elisa-vargas, @david-corneail and I discussed replacing the variant served for the 700 weight font with GT-America-Standard-Bold which is the "appropriate" variant for that weight. However doing this would cause significant changes to the rendered font and cause significant rework of all the existing components that use it.

Additional information

• Since we are already using the Medium variant. the change shouldn't affect the package size at a.
• Here is where the font styles are defined: https://github.com/ithaka/pharos/blob/develop/packages/pharos/src/styles/fonts.css.