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/binary-search/binary-search.d.ts

@@ -0,0 +1,22 @@
+//Typescript type definition for:
+//https://github.com/darkskyapp/binary-search
+declare module 'binary-search' {
+
+function binarySearch<A, B>(
+  haystack: ArrayLike<A>,
+  needle: B,
+  comparator: (a: A, b: B, index?: number, haystack?: A[]) => any,
+  // Notes about comparator return value:
+  // * when a<b the comparator's returned value should be:
+  //   * negative number or a value such that `+value` is a negative number
+  //   * examples: `-1` or the string `"-1"`
+  // * when a>b the comparator's returned value should be:
+  //   * positive number or a value such that `+value` is a positive number
+  //   * examples: `1` or the string `"1"`
+  // * when a===b
+  //    * any value other than the return cases for a<b and a>b
+  //    * examples: undefined, NaN, 'abc'
+  low?: number,
+  high?: number): number; //returns index of found result or number < 0 if not found
+export = binarySearch;
+}