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/CHANGELOG.md

@@ -0,0 +1,43 @@
+# Changelog
+
+## 2.0.0 – Oct 15, 2020
+
+Same as 2.0.0-beta.1.
+
+### BREAKING CHANGES
+
+- Support the font variants used by Shields:
+  - 10px Verdana
+  - bold 10px Verdana
+  - 11px Verdana
+  - bold 11px Helvetica
+
+## 2.0.0-beta.1 – Oct 11, 2020
+
+### BREAKING CHANGES
+
+- Support the font variants used by Shields:
+  - 10px Verdana
+  - bold 10px Verdana
+  - 11px Verdana
+  - bold 11px Helvetica
+
+## 2.0.0-beta.0 – Oct 11, 2020
+
+### BREAKING CHANGES
+
+- Support 10px and 11px Verdana, in normal and bold.
+
+## 1.0.0 – Apr 12, 2018
+
+- Update dependencies.
+- Stable API.
+
+## 0.1.1 – Nov 15, 2018
+
+- Require the JSON file (instead of `loadSync`-ing it) to play well with
+  module bundlers.
+
+## 0.1.0 – Nov 13, 2018
+
+Initial release.