feat: add livecheck for latest-release tracking

Declare the GitHub releases page as the livecheck source (github_latest
strategy) so brew livecheck / brew bump-formula-pr track the latest FetchCord
release.

Author: BlivionIaG

Formula/fetchcord.rb

@@ -37,4 +37,9 @@ def install
   test do
     assert_match "FetchCord version:", shell_output("#{bin}/fetchcord --version")
   end
+
+  livecheck do
+    url "https://github.com/fetchcord/FetchCord/releases"
+    strategy :github_latest
+  end
 end