feat(defs): allow gradientTransform to be passed to gradient definitions

[benjamintd]

Fixes #1098

This PR allows defining gradient transforms as part of the defs for linear gradients.
This fixes the (now stale, but still relevant) associated feature request #1098.

Example usage:

 <Line
        {...commonProperties}
        enableArea={true}
        yScale={{
            type: 'linear',
            stacked: true,
        }}
        curve={select('curve', curveOptions, 'linear')}
        defs={[
            linearGradientDef(
                'gradientA',
                [
                    { offset: 0, color: 'inherit' },
                    { offset: 100, color: 'inherit', opacity: 0 },
                ],
                { gradientTransform: 'rotate(90 0.5 0.5)' }
            ),
        ]}
        fill={[{ match: '*', id: 'gradientA' }]}
    />

The transform attribute follows the SVG spec documented here: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/transform

I added a test and modified the gradient example to show how to use that new property, which for instance allows horizontal gradients on lines:

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[plouc]

LGTM

[plouc]

@benjamintd thank you for your contribution! That's a nice addition to the lib.