Fix markdown warning in parse_product doc comment
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Refactor grid style types and impls
Fixes#16949.
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Prefixed intrinsic size value
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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1355402
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For now, only impls for types like in style::values::generics can be derived.
This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.
Stylo: Add support for grid-template-{rows,columns}
This has the implementation of grid's `<track-list>` nightmare.
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In a similar way to Position, now specified and computed gradients share
a common Gradient type defined in style::values::generics::image.
This allows us to reuse most code for many style traits like ToCss,
HasViewportPercentage and ToComputedValue.
The test changes are the fallout of the disappearance of AngleOrCorner::None,
which align our code to the spec for serialisation, where components that can
be omitted should be omitted.
