chore: add exitFullscreen

Author: Simon he

src/exitFullscreen.ts

@@ -0,0 +1,8 @@
+export function exitFullscreen() {
+  const el: any = (parent as any).documentElement
+  const cfs = el?.cancelFullScreen || el?.webkitCancelFullScreen || el?.mozCancelFullScreen || el?.exitFullScreen;
+  if (cfs)
+    cfs.call(el);
+  else
+    alert("切换失败,可尝试Esc退出")
+}