[File] Add MCAP magic

[Jacob Bandes-Storch]

Hi there, I've submitted a patch adding a magic entry for MCAP log
files (https://mcap.dev - multi-modal pub/sub data, commonly used in
robotics):  https://github.com/file/file/pull/186

I saw a note on GitHub that it's not the preferred way of submitting
patches, however, I also saw that others have had their PRs accepted
through GitHub, so thought I'd try it. Let me know if you'd rather I
submit via email instead.

Thanks!
Jacob Bandes-Storch