Add a isOnlyEmojis() function

[bmarty]

In order to detect when a string contains only Emoji(s), it would be nice if a function isOnlyEmojis(str: String) were provided by the library.

On the plus side, the code will be shared between Android, iOS and Web.

Context: element-hq/element-x-android#1438

Note: on Element Android, we were relying on a third party library to detect such strings.