Intent to prototype: Implement path() on d property on SVG path element

[Boris Chiou]

Hi,

This is a web-compat issue. We would like to make the SVG `d` attribute (on path element) as a presentation attribute. We support the `d` attribute on the SVG path element already, and now we would like to add the `d` property in CSS and so we can render or create animations on the SVG path element by updating its style.

Bug: Bug 1340422
Standard:

1. Presentation attribute (note: check the d property in the table): https://svgwg.org/svg2-draft/styling.html#PresentationAttributes

2. d property: https://svgwg.org/svg2-draft/paths.html#TheDProperty

Platform coverage: All
Preference: "layout.css.d-property.enabled", I would like to enable this on nightly only for now.
DevTools bug: Nope.
Other browsers:

1. Google Chrome supports this

2. Safari: Not supported yet

web-platform-tests:

https://searchfox.org/mozilla-central/source/testing/web-platform/tests/svg/path/property

I will also add some in this folder.

Thanks,

Boris