This callback-based API allows the parser to easily visit each interior node
of the stack when searching for an error repair. It also is a better abstraction
over the stack's DAG implementation than having the public functions for
accessing entries and their successor entries.
Since the capacity is now included in the return value, the buffer
can be reused in the ts_parser__accept function. Also, it's just
cleaner to use Array consistently, rather than a separate buffer
and size.
