Add linter for `fixme` tags

## Feature description

It's not uncommon for software engineers to leave comments with keywords such as FIXME, TODO or TBD during development, for themselves or their close collaborators. Sometimes these keywords are meant to stay, but sometimes they are not and simply fly past reviewers' scrutiny.

There are some tools out there to deal with them, like `pylint`'s [`miscellaneous`](https://pylint.pycqa.org/en/latest/technical_reference/features.html#miscellaneous-checker) style checker or the [`fixme`](https://www.npmjs.com/package/fixme) npm package. For ROS 2 packages, having an `ament` linter to do the job, regardless of the programming language of choice, would be ideal.

## Additional considerations
Following the conventions in this repository, perhaps:
- an `ament_fixme` package, providing [`fixme`](https://www.npmjs.com/package/fixme)-like functionality in a more friendly form
(i.e. no `nodejs`), and
- an `ament_cmake_fixme` package, to hook it all up as an `ament_lint_auto` extension

would do.