Allow multiple top-level nodes

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.

src/runtime/stack.c

@@ -58,6 +58,8 @@ TSTree *ts_stack_reduce(TSStack *stack, TSSymbol symbol, size_t child_count,
   // The child node count is known ahead of time, but some children may be
   // ubiquitous tokens, which don't count.
   for (size_t i = 0; i < child_count; i++) {
+    if (child_count == stack->size)
+      break;
     TSTree *child = stack->entries[stack->size - 1 - i].node;
     if (ts_tree_is_extra(child))
       child_count++;