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/index.js

@@ -0,0 +1,81 @@
+'use strict'
+/**
+ * @module badge-maker
+ */
+
+const _makeBadge = require('./make-badge')
+
+class ValidationError extends Error {}
+
+function _validate(format) {
+  if (format !== Object(format)) {
+    throw new ValidationError('makeBadge takes an argument of type object')
+  }
+
+  if (!('message' in format)) {
+    throw new ValidationError('Field `message` is required')
+  }
+
+  const stringFields = ['labelColor', 'color', 'message', 'label']
+  stringFields.forEach(function (field) {
+    if (field in format && typeof format[field] !== 'string') {
+      throw new ValidationError(`Field \`${field}\` must be of type string`)
+    }
+  })
+
+  const styleValues = [
+    'plastic',
+    'flat',
+    'flat-square',
+    'for-the-badge',
+    'social',
+  ]
+  if ('style' in format && !styleValues.includes(format.style)) {
+    throw new ValidationError(
+      `Field \`style\` must be one of (${styleValues.toString()})`
+    )
+  }
+}
+
+function _clean(format) {
+  const expectedKeys = ['label', 'message', 'labelColor', 'color', 'style']
+
+  const cleaned = {}
+  Object.keys(format).forEach(key => {
+    if (format[key] != null && expectedKeys.includes(key)) {
+      cleaned[key] = format[key]
+    } else {
+      throw new ValidationError(
+        `Unexpected field '${key}'. Allowed values are (${expectedKeys.toString()})`
+      )
+    }
+  })
+
+  // Legacy.
+  cleaned.label = cleaned.label || ''
+
+  return cleaned
+}
+
+/**
+ * Create a badge
+ *
+ * @param {object} format Object specifying badge data
+ * @param {string} format.label (Optional) Badge label (e.g: 'build')
+ * @param {string} format.message (Required) Badge message (e.g: 'passing')
+ * @param {string} format.labelColor (Optional) Label color
+ * @param {string} format.color (Optional) Message color
+ * @param {string} format.style (Optional) Visual style e.g: 'flat'
+ * @returns {string} Badge in SVG format
+ * @see https://github.com/badges/shields/tree/master/badge-maker/README.md
+ */
+function makeBadge(format) {
+  _validate(format)
+  const cleanedFormat = _clean(format)
+  return _makeBadge(cleanedFormat)
+}
+
+module.exports = {
+  makeBadge,
+  ValidationError,
+}