Add in `rm` method

[CMCDragonkai]

Specification

The rm method takes over some of the functionality of rmdir. This rm method was introduced in Node 14.14. I believe some of our uses of EFS may start using rm instead of rmdir. It's probably a good idea to support rm as well.

Additional context

• https://github.com/MatrixAI/js-polykey/pull/266#issuecomment-1017304741 - was confused over why the mkdirExists didn't work, and it's because the FileSystem type in PK is not fulfilled by EFS atm due to the lack of rm.

Tasks

1. Investigate the docs for rm and fsPromises.rm in NodeJS https://nodejs.org/api/fs.html
2. Add in the rm method
3. Keep the old options available in rmdir even though they are deprecated in Node FS.