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/CHANGELOG.md

@@ -0,0 +1,18 @@
+# Changelog
+
+## 1.0.0 – Apr 12, 2019
+
+- Update dependencies.
+- Same API as previous release.
+
+## 0.2.1 – Nov 15, 2018
+
+No external changes.
+
+## 0.2.0 – Nov 13, 2018
+
+- Add `loadConsumerSync()` function.
+
+## 0.1.0 – Nov 13, 2018
+
+Initial release.