Export core/serialization to Trix global

[manubo]

Export core/serialization to the Trix global

This will allow to serialize the contents to HTML, e.g.:

const html = Trix.core.serialization.serializeToContentType(element, "text/html")

or

const html = Trix.core.serialization.serializeToContentType(element.editor.getDocument(), "text/html")

In version 1, serializers where also available via the Trix global, e.g.

const html = Trix.serializers["text/html"](document)

This PR will provide the same functionality for version 2.

[manubo]

The tests failed on the CI with an error that seemed not related to the changes made. Locally, the tests execute successfully.

I pushed an empty commit to trigger CI again and would appreciate, if you could re-approve the test-workflow again, thanks!