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/badge-maker/index.d.ts

@@ -0,0 +1,11 @@
+interface Format {
+  label?: string
+  message: string
+  labelColor?: string
+  color?: string
+  style?: 'plastic' | 'flat' | 'flat-square' | 'for-the-badge' | 'social'
+}
+
+export declare class ValidationError extends Error {}
+
+export declare function makeBadge(format: Format): string