#3372 Added a self event modifier

[samuelgozi]

Added support for a self modifier.
The self modifier only triggers the handler if event.target is the element itself.

Added and updated the tests, and documentation.
I don't know if the ESlint plugin derives the allowed modifiers from this project so I haven't done anything in that regard, but I'll gladly take care of that if needed.

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Tests

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[Rich-Harris]

Thanks! This seems like a good addition (and thanks for including tests and docs — a perfect PR 🤗). I made one small tweak — since this (aka event.currentTarget) is always the element that the handler was attached to, there's no need to pass el around: d9206fb