Support files opened via Files#newBufferedWriter

[adangel]

I noticed, that files opened via Files.newBufferedWriter were not tracked.
This is added now - using sun.nio.ch.FileChannelImpl - the close of the file was already recognized, as internally the file is handled as a FileChannel.

Testing done

Test added in FileDemo. Also added a test for Files.newBufferedReader - but that already worked.

Locally tested with java 11, 17, 21 (mvn clean integration-test)

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[basil]

Thanks for the PR!