appsignal-javascript Frontend improvements

This is a work in progress, feel free to edit it to add/remove things

[ ] Remove redundant packages: merge @appsignal/core and @appsignal/types into @appsignal/javascript.
[ ] Implement missing features such as custom data (support issue)
[ ] Improve error reporting
- [ ] Report the underlying error when reporting an uncaught promise rejection, instead of UncaughtPromiseRejectionError.
- [ ] Report the action in which an error takes place (for example, for a React component, attempt to report the component name)
[ ] Implement integration tests for popular supported integrations
[ ] Implement front-end test setups for popular supported integrations

[ ] https://github.com/appsignal/appsignal-javascript/issues/533 -- We should consider whether we want to implement this. Is there really an use case for initialising AppSignal multiple times? And would there be a benefit, such as making AppSignal easier to use, if we expose it in the global object?
[ ] https://github.com/appsignal/appsignal-javascript/issues/486 -- Since we don't support web vitals metrics via Next.js in the Node.js 3.x integration, this could be a nice way to bring support for it back.
[ ] https://github.com/appsignal/appsignal-javascript/issues/527 -- This would bring the integration's internal logging in line with that of our other integrations, and give our users more control over what is logged by AppSignal. It would also provide a workaround for https://github.com/appsignal/appsignal-javascript/issues/297.