Don't create error node in lexer unless token is completely invalid

Before, any syntax error would cause the lexer to create an error
leaf node. This could happen even with a valid input, if the parse
stack had split and one particular version of the parse stack
failed to parse.

Now, an error leaf node is only created when the lexer cannot understand
part of the input stream at all. When a normal syntax error occurs,
the lexer just returns a token that is outside of the expected token
set, and the parser handles the unexpected token.

src/runtime/stack.h

@@ -107,6 +107,9 @@ void ts_stack_merge_from(Stack *, StackVersion);
 
 void ts_stack_merge(Stack *);
 
+void ts_stack_merge_new(Stack *, StackVersion base_version,
+                        StackVersion new_version);
+
 void ts_stack_renumber_version(Stack *, StackVersion, StackVersion);
 
 StackVersion ts_stack_duplicate_version(Stack *, StackVersion);