Previously, if an error happened right at the beginning of an error
production, the error node would be immediately shifted onto the stack
without calling the error handling function.
The lexer doesn't know the expected symbols, so it doesn't have enough
information to construct error nodes. Now, when it encounters an invalid
character, it returns NULL and the parser builds a correct error node.
Now, the root node of a document is always a document node.
It will often have only one child node which corresponds to the grammar's
start symbol, but not always. Currently, it may have more than one child
if there are ubiquitous tokens such as comments at the beginning of the
document. In the future, it will also be possible be possible to have multiple
for the document to have multiple children if the document is partially parsed.
