function getStyleVariablesContext(context) {
  const sourceCode = context.getSourceCode()
  const df =
    sourceCode.parserServices.getDocumentFragment &&
    sourceCode.parserServices.getDocumentFragment()
  if (!df) {
    return null
  }
  const styles = df.children.filter(
    /** @returns {e is VElement} */
    (e) => isVElement(e) && e.name === 'style'
  )
  if (styles.length === 0) {
    return null
  }
  let ctx = cache.get(styles[0])
  if (ctx) {
    return ctx
  }
  ctx = new StyleVariablesContext(context, styles)
  cache.set(styles[0], ctx)
  return ctx
}
Get the style vars context
@param {RuleContext} context
@returns {StyleVariablesContext | null}