filtered_title and filtered_pages for written_book

[SPGoding]

There are two NBT tags for minecraft:written_book in 21w43a: filtered_title and filtered_pages

I'm not sure the exact snapshot where they were added or what they do.

[misode]

From my diffing, it looks like they were added in 21w07a, would need some investigating

[misode]

Discovered that signs have FilteredText1 - FilteredText4, also added in 21w07a. They're related to a flag textFilteringEnabled that gets sent from the client to the server.