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/badge-cli.js

@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+
+'use strict'
+
+const { namedColors } = require('./color')
+const { makeBadge } = require('./index')
+
+if (process.argv.length < 4) {
+  console.log('Usage: badge label message [:color] [@style]')
+  console.log('Or:    badge label message color [labelColor] [@style]')
+  console.log()
+  console.log('  color, labelColor:')
+  console.log(`    one of ${Object.keys(namedColors).join(', ')}.`)
+  console.log('    #xxx (three hex digits)')
+  console.log('    #xxxxxx (six hex digits)')
+  console.log('    color (CSS color)')
+  console.log()
+  console.log('Eg: badge cactus grown :green @flat')
+  console.log()
+  process.exit()
+}
+
+// Find a format specifier.
+let style = ''
+for (let i = 4; i < process.argv.length; i++) {
+  if (process.argv[i][0] === '@') {
+    style = process.argv[i].slice(1)
+    process.argv.splice(i, 1)
+    continue
+  }
+}
+
+const label = process.argv[2]
+const message = process.argv[3]
+let color = process.argv[4] || ':green'
+const labelColor = process.argv[5]
+
+const badgeData = { label, message }
+if (style) {
+  badgeData.style = style
+}
+
+if (color[0] === ':') {
+  color = color.slice(1)
+  if (namedColors[color] == null) {
+    // Colorscheme not found.
+    console.error('Invalid color scheme.')
+    process.exit(1)
+  }
+  badgeData.color = color
+} else {
+  badgeData.color = color
+  if (labelColor) {
+    badgeData.labelColor = labelColor
+  }
+}
+
+;(() => {
+  try {
+    console.log(makeBadge(badgeData))
+  } catch (e) {
+    console.error(e)
+    process.exit(1)
+  }
+})()