[css-shapes] Allow path() to scale like SVG

[noamr]

A concrete proposal for the concerns raised in #10647.
I think we should diverge path and shape, in a way that path always concretely translates to a path, and maintains its relationship with SVG, and shape is more of a "recipe" for creating a path.

Specifically, path falls short because it uses a scalable format from SVG, but it can't be scaled to the reference-box, because it doesn't have a view-box or a fit, which makes path not very useful for anything responsive.

Suggesting something like the following syntax for path:

path(<fill-rule>?  || <string> || [ [ contain | cover | fill | scale-down  ]? || <number>{2,4} ]?)

Where if a view-box (and potentially a fit) are given, the SVG path would scale to the reference box given those rules.

@LeaVerou thoughts?