feat: add OpenSearch XML endpoint

- Serve /opensearch.xml with configurable base URL
- Browsers can now add gosearch as a search engine from the address bar
- Configurable via [server] base_url or BASE_URL env var
- XML template embedded in the binary via go:embed
- Added base_url to config.example.toml

cmd/searxng-go/main.go

@@ -63,6 +63,7 @@ func main() {
 	mux.HandleFunc("/", h.Index)
 	mux.HandleFunc("/healthz", h.Healthz)
 	mux.HandleFunc("/search", h.Search)
+	mux.HandleFunc("/opensearch.xml", h.OpenSearch(cfg.Server.BaseURL))
 
 	// Serve embedded static files (CSS, JS, images).
 	staticFS, err := views.StaticFS()