[core] Account for character descenders in text-radial-offset

[chloekraw]

GL-Native version of mapbox/mapbox-gl-js#8560

Like text-offset that accepts number or an array of two numbers that adjusts the x and y coordinates, it would be great if text-radial-offset allowed the same functionality. That would account for the extra distance top positioned labels need for characters with descenders (i.e g, p, q):

cc @pozdnyakov @alexshalamov

[stale]

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