Updated implementation for better type info

[WesleyClements]

The current implementation doesn't allow for TypeScript to figure out that Counter extends Transform.
Unfortunately this can't be resolved by simply adding a declaration file since Counter can be called with or without new.
The simplest way I could come up with while changing as little as possible was to refactor the code to use the class syntax.
This unfortunately is a breaking change which results in any code that didn't use new breaking.
As such I have incremented the major version to reflect this.