Add save svg-badge directly to gist

This adds the posibility of saving an SVG badge generated by the same
shields.io dirictly to the gist. Instead of prepering a JSON file to
be sent to their service, we use their library directly, which outputs
an SVG file that we can save to the user’s gist.

Filenames ending in `.svg` will use this library automatically.

Additionally there is a major refactoring where the older `node:http`
library has been swapped out for `fetch`.

Also swap from node 16 to node 20

fixes #24

node_modules/color-convert/component.json

@@ -0,0 +1,9 @@
+{
+  "name": "color-convert",
+  "description": "Plain color conversion functions",
+  "version": "0.5.3",
+  "author": "Heather Arthur <fayearthur@gmail.com>",
+  "repository": "harthur/color-convert",
+  "keywords": ["color", "colour", "rgb"],
+  "scripts": ["index.js", "conversions.js"]
+}