lib: Include fields in ts_node_string output

This allows you to assert about fields in tests. But if your test 
s-expression does *not* include fields, the fields will be stripped from 
the regexp before comparison.

cli/src/tests/parser_test.rs

@@ -721,7 +721,7 @@ fn test_parsing_with_included_ranges_and_missing_tokens() {
     let root = tree.root_node();
     assert_eq!(
         root.to_sexp(),
-        "(program (A (MISSING)) (b) (c) (A (MISSING)) (b) (c))"
+        "(program (A (MISSING a)) (b) (c) (A (MISSING a)) (b) (c))"
     );
     assert_eq!(root.start_byte(), 2);
     assert_eq!(root.child(3).unwrap().start_byte(), 4);