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/index.js

@@ -0,0 +1,45 @@
+module.exports = function(haystack, needle, comparator, low, high) {
+  var mid, cmp;
+
+  if(low === undefined)
+    low = 0;
+
+  else {
+    low = low|0;
+    if(low < 0 || low >= haystack.length)
+      throw new RangeError("invalid lower bound");
+  }
+
+  if(high === undefined)
+    high = haystack.length - 1;
+
+  else {
+    high = high|0;
+    if(high < low || high >= haystack.length)
+      throw new RangeError("invalid upper bound");
+  }
+
+  while(low <= high) {
+    // The naive `low + high >>> 1` could fail for array lengths > 2**31
+    // because `>>>` converts its operands to int32. `low + (high - low >>> 1)`
+    // works for array lengths <= 2**32-1 which is also Javascript's max array
+    // length.
+    mid = low + ((high - low) >>> 1);
+    cmp = +comparator(haystack[mid], needle, mid, haystack);
+
+    // Too low.
+    if(cmp < 0.0)
+      low  = mid + 1;
+
+    // Too high.
+    else if(cmp > 0.0)
+      high = mid - 1;
+
+    // Key found.
+    else
+      return mid;
+  }
+
+  // Key not found.
+  return ~low;
+}