TODO backlinks

Link from GitHub issue #123 to source lines containing TODO(#123): msg.

For example, if source_file.py contains this on line 126:

# TODO(#4): remove comments which no longer have TODOs

Then issue #4 will get a new comment

This issue has 1 outstanding TODOs:

 - [source_file.py:126](https://github.com/j2kun/todo-backlinks/blob/main/entrypoint.py#L126) :  remove comments which no longer have TODOs

This comment was autogenerated by [todo-backlinks](https://github.com/j2kun/todo-backlinks)

The comment will be updated in sync with the code changes, including listing multiple TODOs and deleting the comment when the TODOs are gone. Only works on files in the repository's main branch.