Improve usWeightClass check

[davelab6]

Observed behaviour

uswds/public-sans#119 (comment)

Expected behaviour

If checking a static font that is TTF, not CFF, then the usWeightClass check can be changed to accept and prefer 100 and 200 for Thin and ExtraLight styles' OS/2 table usWeightClass values, and WARN that values of 250 or 275 are deprecated.

But, if checking a TTF variable font, the usWeightClass check should FAIL on 250 and 275 for Thin and ExtraLight styles' OS/2 table usWeightClass values. That way the wght axis values and the OS/2 values will match up.

[laerm0]

Hey @felipesanches , can you take care of this one quickly? It fails on a VF with ExtraLight as 200.