Was having a discussion with @kieferrm and we were thinking the extension should not say it should be the default, but rather indicate that it makes sense and would like to be the default. That way VS Code can ask the user when one of these default extensions is installed when opening a supported file whether it would like to be the default (eg. Luna Paint), but not for "secondary" custom editors where that doesn't make as much sense (eg. Hex Editor).

This will help if we recommend installing Luna Paint from the image viewer (#116120) as the default wouldn't be changed out from under the user.