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/css-unit-converter/README.md

@@ -0,0 +1,26 @@
+# css-unit-converter [![Build Status][ci-img]][ci]
+
+Converts CSS values from one unit to another
+
+[PostCSS]: https://github.com/postcss/css-unit-converter
+[ci-img]:  https://travis-ci.org/andyjansson/css-unit-converter.svg
+[ci]:      https://travis-ci.org/andyjansson/css-unit-converter
+
+
+## Installation
+
+```js
+npm install css-unit-converter
+```
+
+## Usage
+
+
+```js
+var convert = require('css-unit-converter');
+//convert 1 inch to pc
+convert(1, 'in', 'pc'); // 6
+
+//convert 10px to cm with a maximum of 10 decimals
+convert(10, 'px', 'cm', 10); // 0.2645833333
+```