Consider source maps defined in HTTP headers for the Sass custom metric

The [sass.js](https://github.com/HTTPArchive/custom-metrics/blob/main/dist/sass.js#L205) custom metric fetches source SCSS files based on the `sourceMappingURL` declaration in stylesheet bodies. Also look for source map URLs declared in [`SourceMap`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/SourceMap) headers for completeness.