Added a reference of executed/processed modPlugin object in modSystemEvent

### What does it do ?

Adds a new public property (`plugin`) in `modSystemEvent` to reference the currently processed plugin.
(also fixed minor PHPDoc issue)

### Why is it needed ?

In some cases, it could be convenient to have access to the `modPlugin` object, preventing the developer to manually grab it.

### Related issue(s)/PR(s)

* #11129