Moved App Sources into separate files

lib/app_sources/mullvad.dart

@@ -0,0 +1,43 @@
+import 'package:html/parser.dart';
+import 'package:http/http.dart';
+import 'package:obtainium/providers/source_provider.dart';
+
+class Mullvad implements AppSource {
+  @override
+  late String host = 'mullvad.net';
+
+  @override
+  String standardizeURL(String url) {
+    RegExp standardUrlRegEx = RegExp('^https?://$host');
+    RegExpMatch? match = standardUrlRegEx.firstMatch(url.toLowerCase());
+    if (match == null) {
+      throw notValidURL;
+    }
+    return url.substring(0, match.end);
+  }
+
+  @override
+  Future<APKDetails> getLatestAPKDetails(String standardUrl) async {
+    Response res = await get(Uri.parse('$standardUrl/en/download/android'));
+    if (res.statusCode == 200) {
+      var version = parse(res.body)
+          .querySelector('p.subtitle.is-6')
+          ?.querySelector('a')
+          ?.attributes['href']
+          ?.split('/')
+          .last;
+      if (version == null) {
+        throw couldNotFindLatestVersion;
+      }
+      return APKDetails(
+          version, ['https://mullvad.net/download/app/apk/latest']);
+    } else {
+      throw couldNotFindReleases;
+    }
+  }
+
+  @override
+  AppNames getAppNames(String standardUrl) {
+    return AppNames('Mullvad-VPN', 'Mullvad-VPN');
+  }
+}