Include original content in Note

[Knut Magnus Aasrud]

Add a new field original_content (or similarly named,) that contains the raw content of the note, before any link-parsing or front matter stripping is done. This field can be useful in certain contexts, e.g. for returning diagnostic ranges in our language server.

Adding the field will most likely increase our memory usage drastically. It is luckily quite low from the get-go, but keep an eye on it and consider any optimization strategies. Also keep an eye on the benchmarks - especially during serialization.