require url when only format is specified

Signed-off-by: eternal-flame-AD <[email protected]>

js/igv.d.ts

@@ -45,7 +45,8 @@ export type TypeFormatPair<T extends TrackType> = {
     type?: T;
     // if we know the format, type is not required
     format: TrackFormatOf<T>;
-    url?: string;
+    // in this case URL can be anything
+    url: Deferrable<string>;
 } | {
     type?: T;
     format?: TrackFormatOf<T>;