[css-ruby] Propose to use a different syntax for inter-character annotation

I propose to remove inter-character keyword from ruby-position, and handle
annotation as inter-base annotation if the writing-mode of the text
container is orthogonal to that of the base container or ruby container.

The reasons are:
1. We have met some style computation problems for the keyword
inter-character, like the cyclic dependency between writing-mode and
display. And solving those problems seem to be non-trivial.
2. The behavior of an orthogonal text container is never defeined.
3. The keyword inter-character could be confusing, because what it does is
not essentially inter-character. It is inter-base for horizontal line, and
not inter- anything at all for vertical line.

For these reasons, I propose this change.

The only concern for this proposal is that, it will make authors have to
add <rtc> for inter-character annotation, instead of just specifying the
proper ruby-position on some ancestor. But I guess it could be solved if we
backport the writing-mode of an anonymous text container from its first
child. Backporting a property to an anonymous box might probably be less
tricky.

- Xidorn