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

package.json

@@ -2,6 +2,7 @@
   "name": "dynamic-badges-action",
   "version": "1.5.0",
   "description": "A GitHub Action which uses shields.io to create custom badges and uploads the to a gist.",
+  "type": "module",
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1"
@@ -18,6 +19,7 @@
   },
   "homepage": "https://github.com/Schneegans/dynamic-badges-action#readme",
   "dependencies": {
-    "@actions/core": "^1.10.0"
+    "@actions/core": "^1.10.0",
+    "badge-maker": "^3.3.1"
   }
-}
+}