Moves the concept of databases closer to the 3.0 spec, by implementing
the delete functionality, and the method returning current databases to
script.
Testing: WPT tests with new passes
Fixes: The "Implement databases deletion and `databases`" item of
https://github.com/servo/servo/issues/40983
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Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>