bpo-40334: Fix error location upon parsing an invalid string literal (GH-19962)

When parsing a string with an invalid escape, the old parser used to
point to the beginning of the invalid string. This commit changes the new
parser to match that behaviour, since it's currently pointing to the
end of the string (or to be more precise, to the beginning of the next
token).
diff --git a/Parser/pegen/parse_string.h b/Parser/pegen/parse_string.h
index 4f2aa94..cd85bd5 100644
--- a/Parser/pegen/parse_string.h
+++ b/Parser/pegen/parse_string.h
@@ -34,8 +34,8 @@
 } FstringParser;
 
 void _PyPegen_FstringParser_Init(FstringParser *);
-int _PyPegen_parsestr(Parser *, const char *, int *, int *, PyObject **,
-             const char **, Py_ssize_t *);
+int _PyPegen_parsestr(Parser *, int *, int *, PyObject **,
+                      const char **, Py_ssize_t *, Token *);
 int _PyPegen_FstringParser_ConcatFstring(Parser *, FstringParser *, const char **,
                                 const char *, int, int, Token *, Token *,
                                 Token *);