fixes https://github.com/TryGhost/Team/issues/3541
The email preheader, which is only present in the html version of an
email, is also included in the plaintext version of all emails. This
results in all text being duplicated twice in plaintext emails.
- A few users had noticed they get weird content in their excerpts when using e.g. an html block with an iframe
- This use case is common for people embedding podcasts at the top of their posts
refs https://github.com/TryGhost/Toolbox/issues/363
- this shared library is standalone, and it used in various places of
Ghost core, so we can pull it out to keep it easier to reason about
- we also use the `html-to-text` dependency in another package but it's
outdated and could now switch to this new package
