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/xml.js

@@ -0,0 +1,76 @@
+/**
+ * @module
+ */
+
+'use strict'
+
+function stripXmlWhitespace(xml) {
+  return xml.replace(/>\s+/g, '>').replace(/<\s+/g, '<').trim()
+}
+
+function escapeXml(s) {
+  if (typeof s === 'number') {
+    return s
+  } else if (s === undefined || typeof s !== 'string') {
+    return undefined
+  } else {
+    return s
+      .replace(/&/g, '&amp;')
+      .replace(/</g, '&lt;')
+      .replace(/>/g, '&gt;')
+      .replace(/"/g, '&quot;')
+      .replace(/'/g, '&apos;')
+  }
+}
+
+/**
+ * Representation of an XML element
+ */
+class XmlElement {
+  /**
+   * Xml Element Constructor
+   *
+   * @param {object} attrs Refer to individual attrs
+   * @param {string} attrs.name
+   *    Name of the XML tag
+   * @param {Array.<string|module:badge-maker/lib/xml-element~XmlElement>} [attrs.content=[]]
+   *    Array of objects to render inside the tag. content may contain a mix of
+   *    string and XmlElement objects. If content is `[]` or ommitted the
+   *    element will be rendered as a self-closing element.
+   * @param {object} [attrs.attrs={}]
+   *    Object representing the tag's attributes as name/value pairs
+   */
+  constructor({ name, content = [], attrs = {} }) {
+    this.name = name
+    this.content = content
+    this.attrs = attrs
+  }
+
+  /**
+   * Render the XML element to a string, applying appropriate escaping
+   *
+   * @returns {string} String representation of the XML element
+   */
+  render() {
+    const attrsStr = Object.entries(this.attrs)
+      .map(([k, v]) => ` ${k}="${escapeXml(v)}"`)
+      .join('')
+    if (this.content.length > 0) {
+      const content = this.content
+        .map(function (el) {
+          if (el instanceof XmlElement) {
+            return el.render()
+          } else {
+            return escapeXml(el)
+          }
+        })
+        .join(' ')
+      return stripXmlWhitespace(
+        `<${this.name}${attrsStr}>${content}</${this.name}>`
+      )
+    }
+    return stripXmlWhitespace(`<${this.name}${attrsStr}/>`)
+  }
+}
+
+module.exports = { escapeXml, stripXmlWhitespace, XmlElement }