[Dev-luatex] Getters and setters for pdf_link_state whatsits

Hi LuaTeX team, I recently tried to interact with pdf_link_state whatsits from Lua and the current situation is a bit odd: If `n` is a pdf_link_state node, then `n.value` can be assigned to set the parmeter of the pdf_link_state node, but reading `n.value` results in nil. When using node.direct, neither getfield nor setfield for on the `value` field. Assuming that this is just an oversight I attach a patch to at all `value` to all the usual getters and setters. Best, Marcel