[css-env][css-link-params] Function to access stylesheet URL data

[Crissov]

In #6408, I apparently misremembered the existence of a proposal for a pseudo-function similar to attr(), var() or env(), so I will hereby submit one. ;)

The Link Parameters module specifies several ways to pass parameters
from a stylesheet (link-parameters property, url() function) or a random URL (#param()), e.g. embedded within an HTML document,
to a target resource that understands CSS Custom Properties, i.e. usually a stylesheet or an SVG image.

This proposal is about a more generic way at the receiving end:
A new pseudo-function, say arg() (TBB), gives access to the query parameters used in the URL to include the stylesheet (or HTML/SVG/… document in case of inline styles). It works mostly identical to attr(), just is the same for all elements.

Variant: Instead of the URL parameters of the stylesheet, arg() accesses the query component of the host document. For SVG images with inline styles, the end result is the same.

/* in example.svg?color=green */
:root {
--foo: arg(color); /* green */
}
#test {
  fill: var(--foo);
  stroke-color: arg(color);
}