The previously existing toBuilder method defined on the CraftMetaBook
would, no matter if called on a writable or written book, return a
builder targetting a writable book, in which complex components, such as
hover or click text are not allowed.
The builer hence serializes the page components using LEGACY_UXRC to
ensure only colour may be passed. While this works as intented for
writable books, the builder fails to fully support the complex
components that may be used in a written book.
This commit implemements a child class of the CraftMetaBookBuilder, the
CraftMetaBookSignedBuilder, which builds to a CraftMetaBookSigned
instance and hence serializes the pages to json.
This builder instance is automatically supplied when calling toBuilder
on a CraftMetaBookSigned instance.
Resolves: #6296
68db0c9853