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

composer.json

@@ -22,8 +22,8 @@
         "php": "^8.3",
         "illuminate/contracts": "^10.0 || ^11.0 || ^12.0",
         "spatie/laravel-package-tools": "^1.16",
-        "symfony/css-selector": "7.4.x-dev",
-        "symfony/dom-crawler": "7.4.x-dev"
+        "symfony/css-selector": "^6.4 || ^7.0 || ^8.0",
+        "symfony/dom-crawler": "^6.4 || ^7.0 || ^8.0"
     },
     "require-dev": {
         "orchestra/testbench": "^8.22 || ^9.0 || ^10.0",