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-name/README.md

@@ -0,0 +1,11 @@
+A JSON with color names and its values. Based on http://dev.w3.org/csswg/css-color/#named-colors.
+
+[![NPM](https://nodei.co/npm/color-name.png?mini=true)](https://nodei.co/npm/color-name/)
+
+
+```js
+var colors = require('color-name');
+colors.red //[255,0,0]
+```
+
+<a href="LICENSE"><img src="https://upload.wikimedia.org/wikipedia/commons/0/0c/MIT_logo.svg" width="120"/></a>