Currently PM doesn't understand .heic files at all. This is a pity, but we could already get quite bit further if (ordered by priority):
1) PM joins existing .xmp files with .heic files (so if there is a .xmp file for a .heic, it is hidden in the contact sheet).
2) PM reads the metadata for the .heic file from existing .xmp files and shows it as such
3) PM allows writing metadata to .heic in the .xmp file
Implementing this shouldn't be too hard, I think, since this is basically how it is done for .mov files already.
Note: Photoshop (Camera Raw) writes edits to .heic files in the .xmp file as well.