AK: Add String::number() for creating a String from a number.

Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.

AK/Tests/TestQueue.cpp

@@ -28,7 +28,7 @@ int main()
     EXPECT(strings.is_empty());
 
     for (int i = 0; i < 10000; ++i) {
-        strings.enqueue(String::format("%d", i));
+        strings.enqueue(String::number(i));
         EXPECT_EQ(strings.size(), i + 1);
     }