tools: ynl: move private definitions to a separate header

ynl.h has a growing amount of "internal" stuff, which may confuse
users who try to take a look at the external API. Currently the
internals are at the bottom of the file with a banner in between,
but this arrangement makes it hard to add external APIs / inline
helpers which need internal definitions.

Move internals to a separate header.

Link: https://lore.kernel.org/r/20231202211225.342466-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>