Refactor and improve external URL checks

- Move external URL checks to its own module under `tests/`. This
  separates them from integration test, addressing long runs and
  frequent failures that led to ignoring test results.
- Move `check-desktop-runtime-errors` to `tests/checks` to keep all
  test-related checks into one directory.
- Replace `ts-node` with `vite` for running
  `check-desktop-runtime-errors` to maintain a consistent execution
  environment across checks.
- Implement a timeout for each fetch call.
- Be nice to external sources, wait 5 seconds before sending another
  request to an URL under same domain. This solves rate-limiting issues.
- Instead of running test on every push/pull request, run them only
  weekly.
- Do not run tests on each commit/PR but only scheduled (weekly) to
  minimize noise.
- Fix URLs are not captured correctly inside backticks or parenthesis.

README.md

@@ -76,6 +76,12 @@
       src="https://github.com/undergroundwires/privacy.sexy/workflows/checks.desktop-runtime-errors/badge.svg"
     />
   </a>
+  <a href="https://github.com/undergroundwires/privacy.sexy/actions/workflows/checks.external-urls.yaml" target="_blank" rel="noopener noreferrer">
+    <img
+      alt="Status of external URL checks"
+      src="https://github.com/undergroundwires/privacy.sexy/workflows/checks.external-urls/badge.svg"
+    />
+  </a>
   <!-- Release -->
   <br />
   <a href="https://github.com/undergroundwires/privacy.sexy/actions/workflows/release.git.yaml" target="_blank" rel="noopener noreferrer">