Set tracker’s fetch call priority to low

Analytic is important activity but not as important as main business
logic. Setting low priority doesn't mean the fetch call will be more
likely to be discarded etc. it's just a hint to the browser.

src/tracker/index.js

@@ -166,6 +166,7 @@
           ...(typeof cache !== 'undefined' && { 'x-umami-cache': cache }),
         },
         credentials: 'omit',
+        priority: 'low',
       });
 
       const data = await res.json();