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/lexer.h

@@ -7,9 +7,19 @@ extern "C" {
 
 #include "tree_sitter/parser.h"
 
+typedef struct {
+  TSSymbol symbol;
+  TSLength padding;
+  TSLength size;
+  bool is_fragile;
+  int32_t first_unexpected_character;
+} TSLexerResult;
+
 void ts_lexer_init(TSLexer *);
 void ts_lexer_set_input(TSLexer *, TSInput);
 void ts_lexer_reset(TSLexer *, TSLength);
+void ts_lexer_start(TSLexer *, TSStateId);
+void ts_lexer_finish(TSLexer *, TSLexerResult *);
 
 #ifdef __cplusplus
 }