Auto merge of #14174 - emilio:font-provider, r=Manishearth

style: Refactor and add infrastructure for font metrics in style.

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This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.

Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.

Then we should make the font provider mandatory, and implement the missing stylo
bits.

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