Empty productions are supposed to be assigned the start attributes
of the lookahead token. Currently, this happens by assigning above
the current stack position when the token it read.
This fails in a situation where we first reduce an empty production
higher up in the stack, and then again reduce an empty production
lower in the stack, without consuming the lookahead token in the
meantime.
Fix this by moving the assignment into the reduction phase. We
also need to do this for error productions, which are effectively
empty.
Adds support for PHP 8 attributes, represented using `AttrGroup` nodes
containing `Attribute` nodes. The `attrGroup` subnode is added to all
nodes that can have attributes.
This is still missing FPPP support.
Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>
