message: fix(core): avoid depending on external types
  inline UrlNode, so that public API of stylable-assets.ts won't require consumers to install
  our type checking verifies the upstream type matches ours.
diff: import path from 'path';
  import type * as postcss from 'postcss';
  -import cssSelectorTokenizer, { AnyValueNode, UrlNode } from 'css-selector-tokenizer';
  +import cssSelectorTokenizer from 'css-selector-tokenizer';
  const { parseValues, stringifyValues } = cssSelectorTokenizer;
  +export interface UrlNode {
  + type: 'url';
  + url: string;
  + stringType?: string;
  + name?: string;
  + before?: string;
  + after?: string;
  + innerSpacingBefore?: string;
  + innerSpacingAfter?: string;
  +}
  +
  export type OnUrlCallback = (node: UrlNode) => void;
  export function collectAssets(ast: postcss.Root) {
  @@ -63,7 +74,7 @@ export function processDeclarationUrls(
  }
  }
  -function findUrls(node: AnyValueNode, onUrl: OnUrlCallback) {
  +function findUrls(node: cssSelectorTokenizer.AnyValueNode, onUrl: OnUrlCallback) {
  switch (node.type) {
  case 'value':
  node.nodes.forEach((child) => findUrls(child, onUrl));
  