Handle specialized serialization of <position> in basic shapes
Fixes#13083
We temporarily broke basic-shape serialization in #13042 when 4-value positions were implemented, since I didn't want to increase the scope of that PR too much.
This fixes it.
r? @SimonSapin
cc @canaltinova
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Prefer length and percentage for word spacing
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The goal is to make use of `LengthOrPercentage` for word spacing in `ShapingOptions`, but since it makes use of `f32` which doesn't implement `Hash`, we're going for `NotNan<f32>` from [ordered-float](https://github.com/reem/rust-ordered-float/), which supports hashing. Instead of implementing `Hash` for `LengthOrPercentage` and thereby the inner types like `CSSFloat`, `CalcLengthOrPercentage`, etc., we convert it to `(Au, NotNan<f32>)`.
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