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/color-convert/index.js

@@ -0,0 +1,92 @@
+var conversions = require("./conversions");
+
+var convert = function() {
+   return new Converter();
+}
+
+for (var func in conversions) {
+  // export Raw versions
+  convert[func + "Raw"] =  (function(func) {
+    // accept array or plain args
+    return function(arg) {
+      if (typeof arg == "number")
+        arg = Array.prototype.slice.call(arguments);
+      return conversions[func](arg);
+    }
+  })(func);
+
+  var pair = /(\w+)2(\w+)/.exec(func),
+      from = pair[1],
+      to = pair[2];
+
+  // export rgb2hsl and ["rgb"]["hsl"]
+  convert[from] = convert[from] || {};
+
+  convert[from][to] = convert[func] = (function(func) { 
+    return function(arg) {
+      if (typeof arg == "number")
+        arg = Array.prototype.slice.call(arguments);
+      
+      var val = conversions[func](arg);
+      if (typeof val == "string" || val === undefined)
+        return val; // keyword
+
+      for (var i = 0; i < val.length; i++)
+        val[i] = Math.round(val[i]);
+      return val;
+    }
+  })(func);
+}
+
+
+/* Converter does lazy conversion and caching */
+var Converter = function() {
+   this.convs = {};
+};
+
+/* Either get the values for a space or
+  set the values for a space, depending on args */
+Converter.prototype.routeSpace = function(space, args) {
+   var values = args[0];
+   if (values === undefined) {
+      // color.rgb()
+      return this.getValues(space);
+   }
+   // color.rgb(10, 10, 10)
+   if (typeof values == "number") {
+      values = Array.prototype.slice.call(args);        
+   }
+
+   return this.setValues(space, values);
+};
+  
+/* Set the values for a space, invalidating cache */
+Converter.prototype.setValues = function(space, values) {
+   this.space = space;
+   this.convs = {};
+   this.convs[space] = values;
+   return this;
+};
+
+/* Get the values for a space. If there's already
+  a conversion for the space, fetch it, otherwise
+  compute it */
+Converter.prototype.getValues = function(space) {
+   var vals = this.convs[space];
+   if (!vals) {
+      var fspace = this.space,
+          from = this.convs[fspace];
+      vals = convert[fspace][space](from);
+
+      this.convs[space] = vals;
+   }
+  return vals;
+};
+
+["rgb", "hsl", "hsv", "cmyk", "keyword"].forEach(function(space) {
+   Converter.prototype[space] = function(vals) {
+      return this.routeSpace(space, arguments);
+   }
+});
+
+module.exports = convert;