Add `FileSystem` to the changelog (if we use the changelog)

I'm not sure if this is actually for this UXP version, but I think the addition of the `FileSystem` APIs needs to be in this changelog (as at least from a 3P perspective, this is in "the new release").

_Originally posted by @pklaschka in https://github.com/AdobeDocs/uxp/pull/61#discussion_r996862147_

Check whether this UXP release has a changelog and review its existing entries before deciding where FileSystem belongs. Confirm the APIs are part of the release covered by the changelog; done means the changelog is updated if appropriate, or the issue is resolved with that decision documented.

Written by the indexing model from the issue text.