Auto merge of #17704 - mantaroh:interpolate-stroke, r=nox

Add animation value related with stroke-dasharray / stroke-dashoffset / stroke-width.

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1369614

This patch will:
 * Add animation value conversion of LengthOrPercentage in order to interpolate length and number.
 * Add animation value of Vec<LengthOrPercentage> in order to interpolate the stroke-dasharray.

Spec is as follow:
https://svgwg.org/svg2-draft/painting.html#StrokeDashing

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There are tests for these changes, a test case will be landed in wpt in https://bugzilla.mozilla.org/show_bug.cgi?id=1369614.

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