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/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "css-unit-converter",
+  "version": "1.1.2",
+  "description": "Converts CSS values from one unit to another",
+  "main": "index.js",
+  "scripts": {
+    "test": "node test/test.js"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/andyjansson/css-unit-converter.git"
+  },
+  "keywords": [
+    "css",
+    "value",
+    "unit",
+    "converter",
+    "convert"
+  ],
+  "author": "Andy Jansson",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/andyjansson/css-unit-converter/issues"
+  },
+  "homepage": "https://github.com/andyjansson/css-unit-converter",
+  "devDependencies": {
+    "tape": "^4.6.3"
+  }
+}