Pass document direction info to font shaper by default

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Fixes #1472

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These changes feel right to me, but an opinion from somebody that actually publishes stuff in directions other than TTB-LTR would be welcome. @ctrlcctrlv @khaledhosny @simoncozens ...

As of this PR:

• When setting the direction option of a document class, the font.direction setting's default value is changed to match.
• When setting the direction of the current frame, the font.direction settings current value is changed to match.

Does this feel right over the prior behavior of being expected to set this manually (via either \set[parameter=font.direction] or \font[direction=]) in addition to setting the class or frame properties so things other than shaped text advance correctly?

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Baring feedback I'm moving ahead with what makes sense to me here, but I'm happy to review again if this turns out to be unexpected behavior.