lua virtual category

[nia <nia%NetBSD.org@localhost>]

hi,

i would like to add a virtual category for "lua", similar to
the ones for "python", "gnome", etc.

initially i thought it could just be added to package makefiles,
but pkglint warns about an invalid category being used. is more
infrastrucutre work needed to add categories, or do they just
need to be added to pkglint?

thanks