With the `OpObserver` moving to the transaction rather than being passed
in to the `Transaction::commit` method we have needed to add a way to
get the observer back out of the transaction (via
`Transaction::observer` and `AutoCommit::observer`). This `Observer`
type is then used to handle patch generation logic. However, there are
cases where we might not want an `OpObserver` and in these cases we can
execute various things fast - so we need to have something like an
`Option<OpObserver>`. In order to track the presence or otherwise of the
observer at the type level introduce
`automerge::transaction::observation`, which is a type level `Option`.
This allows us to efficiently choose the right code paths whilst
maintaining correct types for `Transaction::observer` and
`AutoCommit::observer`
JS packaging is complicated and testing it manually is irritating. Add a
tool in `automerge-js/e2e` which stands up a local NPM registry and
publishes the various packages to that registry for use in automated and
manual tests. Update the test script in `scripts/ci/js_tests` to run the
tests using this tool
