Configure Puppeteer to not download Chrome headless shell

Nowadays Chrome has a built-in (new) headless mode in the regular
binary, but before that time there was an old headless mode that was
essentially a separate binary [1]. We don't use the latter, but it turns
out that Puppeteer downloads it automatically if it's not explicitly
skipped, which is wasteful because it costs extra time and resources for
each `npm install` invocation.

This commit therefore skips downloading Chrome headless shell explictly,
which results in the local runtime of `npm install` going from 10.125
seconds to 8.998 seconds (which can't hurt in e.g. GitHub Actions).

[1] https://developer.chrome.com/blog/chrome-headless-shell.

.puppeteerrc.json

@@ -2,6 +2,9 @@
   "chrome": {
     "skipDownload": false
   },
+  "chrome-headless-shell": {
+    "skipDownload": true
+  },
   "firefox": {
     "skipDownload": false,
     "version": "nightly"