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/anafanafo/index.js

@@ -0,0 +1,22 @@
+'use strict'
+
+const { createConsumer } = require('char-width-table-consumer')
+
+const consumers = {
+  '10px Verdana': createConsumer(require('./data/verdana-10px-normal.json')),
+  'bold 10px Verdana': createConsumer(require('./data/verdana-10px-bold.json')),
+  '11px Verdana': createConsumer(require('./data/verdana-11px-normal.json')),
+  'bold 11px Helvetica': createConsumer(
+    require('./data/helvetica-11px-bold.json')
+  ),
+}
+
+module.exports = function measure(text, { font, ...rest }) {
+  const consumer = consumers[font]
+  if (!consumer) {
+    throw Error(
+      `Unknown font "${font}", expected ${Object.keys(consumers).join(', ')}`
+    )
+  }
+  return consumer.widthOf(text, { ...rest })
+}