Add isBrowserTabFocused, fixed is tests

snippets/isBrowserTabFocused.md

@@ -0,0 +1,13 @@
+### isBrowserTabFocused
+
+Returns `true` if the browser tab of the page is focused, `false` otherwise.
+
+Use the `Document.hidden` property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
+
+```js
+const isBrowserTabFocused = () => !document.hidden;
+```
+
+```js
+isBrowserTabFocused(); // true
+```