Fix sourcemappings #742

Sourcemapping was added in #275, but is still broken. toucan-js does provide an example that has sourcemap support: https://github.com/robertcepa/toucan-js/blob/master/examples/wrangler-basic/package.json. Judging by the difference in the example and what we have, the main thing is we need to create an actual release in Sentry and tie not only the sourcemap but the worker's deployment to it as well.