Consider starting a local node server for testing

A recent commit moved the svg-sprite to use an externally referenced file, great for caching. However for SVG sandboxing reasons, that means the sprite demo page can only be tested when served from `http://` or `https://`, not from `file://`.

Can we leverage Node here, so that when you run `npm start` not only does it build changes when you make them, but it serves results on something like http://127.0.0.1:5000?

Start by inspecting the npm start entry point and how the current SVG build is launched. Run npm start to confirm the existing behavior, then verify that the demo is served at a local http://127.0.0.1:5000-style URL while changes continue to build. Done means the generated demo can be tested without opening it through file://.

Written by the indexing model from the issue text.