Fix packaging, timezone handling and HTML cleaning bugs

- Replace symfony 7.4.x-dev constraints with stable ranges so the
  package installs with minimum-stability stable
- Add the Cision facade referenced in composer.json and a config file
  (slug, timezone); the constructor now falls back to config('cision.slug')
- Convert feed timestamps with a real timezone instead of a hardcoded +2h,
  which was off by an hour during winter time
- Return false from fetchContent() when the request fails or the page
  has no article/heading, instead of throwing
- Fix italic paragraph detection, which never matched
- Fix literal '\n' in HtmlCleaner whitespace cleanup
- Normalize empty feed elements to empty strings
- Update README and add tests for the above

config/cision.php

@@ -0,0 +1,16 @@
+<?php
+
+return [
+
+    /*
+     * The Cision newsroom slug, e.g. "example-company" in
+     * https://news.cision.com/se/example-company
+     */
+    'slug' => env('LARAVEL_CISION_FEED_SLUG'),
+
+    /*
+     * Timezone used when formatting the "created_at" value of feed items.
+     */
+    'timezone' => env('LARAVEL_CISION_FEED_TIMEZONE', 'Europe/Stockholm'),
+
+];