feat(cli): support snapshot testing with --update flag

This PR adds an `--update` flag to the `tree-sitter test` command, which adds the ability to replace the _expected_ output in the corpus.txt with the _actual_ output produced by the parser, that is, we can now simply use this `--update` flag to write all the corresponding parser output back to the corpus.txt, and we just need to check the output without typing its actual sexp.

- use the same output format as `tree-sitter parse`, except there won't be any position information printed.
- the corpus.txt won't be touched if there's no difference between the _expected_ output and the _actual_ output in that file.
- if there're differences between _expected_ and _actual_, _expected_ will be replaced by _actual_ and the whole file will be reformatted, i.e., all the output sexp will be formatted just like the output from `tree-sitter parse` and all the delimiters `===`/`---` will be normalized as 80-column long.
- this flag also works with `--filter` flag.

cli/src/tests/corpus_test.rs

@@ -381,7 +381,7 @@ fn flatten_tests(test: TestEntry) -> Vec<(String, Vec<u8>, String, bool)> {
                 }
                 result.push((name, input, output, has_fields));
             }
-            TestEntry::Group { mut name, children } => {
+            TestEntry::Group { mut name, children, .. } => {
                 if !prefix.is_empty() {
                     name.insert_str(0, " - ");
                     name.insert_str(0, prefix);