Improve URL checks to reduce false-negatives

This commit improves the URL health checking mechanism to reduce false
negatives.

- Treat all 2XX status codes as successful, addressing issues with codes
  like `204`.
- Exclude URLs within Markdown inline code blocks.
- Send the Host header for improved handling of webpages behind proxies.
- Improve formatting and context for output messages.
- Fix the defaulting options for redirects and cookie handling.
- Add URL exclusion support for non-responsive URLs.
- Update the user agent pool to modern browsers and platforms.
- Improve CI/CD workflow to respond to modifications in the
  `test/checks/external-urls` directory, offering immediate feedback on
  potential impacts to the external URL test.
- Add support for randomizing TLS fingerprint to mimic various clients
  better, improving the effectiveness of checks. However, this is not
  fully supported by Node.js's HTTP client; see nodejs/undici#1983 for
  more details.
- Use `AbortSignal` instead of `AbortController` as more modern and
  simpler way to handle timeouts.

.github/workflows/checks.external-urls.yaml

@@ -3,6 +3,9 @@ name: checks.external-urls
 on:
   schedule:
     - cron: '0 0 * * 0' # at 00:00 on every Sunday
+  push:
+    paths:
+      - tests/checks/external-urls/**
 
 jobs:
   run-check: