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/badge-maker/lib/make-badge.js

@@ -0,0 +1,63 @@
+'use strict'
+
+const { normalizeColor, toSvgColor } = require('./color')
+const badgeRenderers = require('./badge-renderers')
+const { stripXmlWhitespace } = require('./xml')
+
+/*
+note: makeBadge() is fairly thinly wrapped so if we are making changes here
+it is likely this will impact on the package's public interface in index.js
+*/
+module.exports = function makeBadge({
+  format,
+  style = 'flat',
+  label,
+  message,
+  color,
+  labelColor,
+  logo,
+  logoPosition,
+  logoWidth,
+  links = ['', ''],
+}) {
+  // String coercion and whitespace removal.
+  label = `${label}`.trim()
+  message = `${message}`.trim()
+
+  // This ought to be the responsibility of the server, not `makeBadge`.
+  if (format === 'json') {
+    return JSON.stringify({
+      label,
+      message,
+      logoWidth,
+      // Only call normalizeColor for the JSON case: this is handled
+      // internally by toSvgColor in the SVG case.
+      color: normalizeColor(color),
+      labelColor: normalizeColor(labelColor),
+      link: links,
+      name: label,
+      value: message,
+    })
+  }
+
+  const render = badgeRenderers[style]
+  if (!render) {
+    throw new Error(`Unknown badge style: '${style}'`)
+  }
+
+  logoWidth = +logoWidth || (logo ? 14 : 0)
+
+  return stripXmlWhitespace(
+    render({
+      label,
+      message,
+      links,
+      logo,
+      logoPosition,
+      logoWidth,
+      logoPadding: logo && label.length ? 3 : 0,
+      color: toSvgColor(color),
+      labelColor: toSvgColor(labelColor),
+    })
+  )
+}