Message: Rename 'interface' prop because of syntax highlighting
Some syntax highlighters, in particular Gerrit's, incorrectly handle
code that uses a property named 'interface' (which is a PHP keyword).
https://phabricator.wikimedia.org/F55884039
Rename it to avoid problems.
Message isn't '@stable to extend', so this is not a breaking change.
I couldn't find any extensions that refer to it anyway.
Change-Id: I6676421222921b7bb1aeeefef79dd06072c1c2ac