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/char-width-table-consumer/src/index.js

@@ -0,0 +1,16 @@
+'use strict'
+
+const CharWidthTableConsumer = require('./consumer')
+
+const {
+  create: createConsumer,
+  load: loadConsumer,
+  loadSync: loadConsumerSync,
+} = CharWidthTableConsumer
+
+module.exports = {
+  createConsumer,
+  loadConsumer,
+  loadConsumerSync,
+  CharWidthTableConsumer,
+}