xref provides support for jump-to-definition, but no support for
jump-to-interface. The latter would jump to the header or interface
file for an identifier. This functionality exists in some packages
already because it is particularly important in some languages, and I'm
looking for suggestions on how to expose it through xref.
I think that it's a bad idea to add language specific commands to xref.
The next thing is that we have commands for jump-to-variable and
jump-to-constant and jump-to-macro ...
It would be better to leave those commands in the language specific mode
with the C-c prefix.