`url()`s containing file paths are not distinguishable from other arbitrary `Word` nodes

Start by reproducing the documented postcss-values-parser examples for absolute and relative paths, then inspect how Word, Func, and isUrl are assigned during parsing. Compare the result with css-tree's Url nodes and the Word.md URL-handling documentation; done means relative url(/images/image.png) is distinguishable from arbitrary Word values in the parser output.

Expected Behavior / Situation

It is common in CSS to specify URLs as an absolute or relative path without a scheme, e.g. url(/images/image.png). css-tree parses these as unambiguous Url nodes:

> const csstree = await import("css-tree")
> csstree.parse("url(https://example.com/image.png)", { context: "value" }).children.head.data
{ type: 'Url', loc: null, value: 'https://example.com/image.png' }
> csstree.parse("url(/images/image.png)", { context: "value" }).children.head.data
{ type: 'Url', loc: null, value: '/images/image.png' }

It was my expectation that postcss-values-parser would yield Func nodes for both of these inputs, per this documentation.

Actual Behavior / Situation

In reality, postcss-values-parser yields Word nodes in both cases:

> const valuesParser = (await import("postcss-values-parser")).parse
> valuesParser.parse("url(https://example.com/image.png)").nodes[0]
<ref *1> Word {
  raws: {},
  value: 'https://example.com/image.png',
  source: [Object],
  isColor: false,
  isHex: false,
  isUrl: true,
  isVariable: false,
  type: 'word',
  parent: [Root],
  Symbol(isClean): false,
  Symbol(my): true
}
> valuesParser("url(/images/image.png)").nodes[0]
<ref *1> Word {
  raws: {},
  value: '/images/image.png',
  source: [Object],
  isColor: false,
  isHex: false,
  isUrl: false,
  isVariable: false,
  type: 'word',
  parent: [Root],
  Symbol(isClean): false,
  Symbol(my): true
}

In the first case, isUrl is set as expected, but in the second case, since the value is not actually a valid URL (at least according to is-url-superb, which just uses new URL()—TBH just using URL.parse() or URL.canParse() directly without the dependency would make more sense iff requiring a minimum of Node 18 is possible, IMHO), it remains false and the node is not distinguishable from other non-URL values despite being unambiguous in the css-tree parse.

Modification Proposal

I suggest that the parser should behave according to the documentation, which would result in a parse something like this (hypothetical, not real output):

> valuesParser("url(/images/image.png)").nodes[0]
<ref *1> Func {
  raws {},
  value: '',
  nodes: [
    Word {
      raws: {},
      value: '/images/image.png',
      source: [Object],
      isColor: false,
      isHex: false,
      isUrl: false,
      isVariable: false,
      type: 'word',
      parent: [Circular *1],
      Symbol(isClean): false,
      Symbol(my): true
    }
  ],
  source: [Object],
  isColor: false,
  isVar: false,
  name: 'url',
  params: '',
  type: 'func',
  parent: [Root],
  Symbol(isClean): false,
  Symbol(my): true
}

Or, failing that, set isUrl to true for Word nodes created from css-tree Url nodes regardless of whether the contents look like a URL.

[!NOTE]
My use case is similar to that of rollup-plugin-styler's URL loader, which parses URL and partial URL values in order to perform lookup resolution for bundling.