Improve `nseventforwarder`

[MarkusPettersson98]

This PR improves the nseventforwarder implementation in the following ways:

• Use clearly named variables
• Add some more documentation to distinguish what's happening on the nodejs vs rust side
• Get rid of the static Mutex, effectively allowing multiple nsevent-callback to be running at once. Because why not

This is not critical in any way, but especially the static Mutex bothered me since it didn't seem necessary :-)

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This change is