Add support for addListener on iOS

[Sharcoux]

We released on @cantoo/text-to-speech if you want to test it. That release is based on version 5.1, though, while this PR is on top of master (v6)

Pull request checklist

Please check if your PR fulfills the following requirements:

• The changes have been tested successfully.

[robingenz]

I am closing this PR because the changes cannot be merged and no issue is linked.

[Sharcoux]

My bad, the PR is wrong indeed. I'll fix it. However, it really fixes a problem:

#140 (for iOS) and #132 (for iOS)

Basically, this makes onRangeStart work for iOS

[Sharcoux]

I merged the wrong branch (twice). I'll just open a new MR explaining the issue better and with the correct branch as a source so you don't get the code from Cantoo fork, but just what needs to be updated here.