24 and 40 spriteSize in the js library.

Hello, thank you for the awesome emoji toolkit.

Just wondering if there will be a change in the js library anytime soon to support 24x24 and 40x40 sprites? As I dig into the code, when sprites are turned on, the spriteSize is just constrained with `'32'` and `'64'` and cannot be overridden during any transformation methods like `toImage`.

In the PHP package, the client config can be set that's why we were able to use sizes other than `'32'` and `'64'` and use the `'24'` and `'40'` sprites that are included in `emoji-assets`. Hoping this could be possible in the JS library too.

Another question also, is there any plan to support ESM?

Happy to provide a PR also.