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/index.d.ts

@@ -0,0 +1,10 @@
+declare module 'css-unit-converter' {
+  export type CSSUnits = 'px' | 'cm' | 'mm' | 'in' | 'pt' | 'pc' | 'deg' | 'grad' | 'rad' | 'turn' | 's' | 'ms' | 'Hz' | 'kHz' | 'dpi' | 'dpcm' | 'dppx';
+
+  export default function (
+    value: number,
+    sourceUnit: CSSUnits,
+    targetUnit: CSSUnits,
+    precision?: number
+  ): number;
+}