Add papis-citar.el: Citar backend for Papis

[jabarszcz]

I've written this package adding Citar compatibility (separate package because of the additional dependency on Citar). I'll be happy to make some changes, either in this PR or separately, notably:

• Adding some details to the README.
• Adding tests.
• I've included variations of papis.el functions (papis--cmd, papis-query, papis--ensured-notes-path) to minimize the number of process calls, avoid temporary file creation, and argument format/parsing round-trips. However, I've refrained from chasing the API changes for now, and so I've left both versions, but they should probably be merged.
• I was also hoping to clarify the project's license, so I could add it to papis-citar.el as well.