Add support for generic event types

[Nikaple]

This PR aims to provide a backward-compatible change on index.d.ts, allowing generic event types. Please refer to this TS playground for more info.

[Nikaple]

Thanks for reviewing! Is there any chances that these changes could be released?

[Simon-Laux]

I'm just another user xD

lets hope @scottcorgan looks at it

so code looks good, just wanted to do the same then found your pr, I merged it to a fork of this repo.
Also there is always the possibility to either release a fork on npm or just use a git repo like this:

npm install https://github.com/Simon-Laux/tiny-emitter