[css-writing-modes] Support vertical CJK + ttb Arabic (rtl scripts generally)

In Issue 2754 I proposed a new value horizontal-tb-rl to writing-mode.

Now I would like to bring up an improved proposal to replace the older one:

Instead of adding a new value horizontal-tb-rl to writing-mode, it would be better to add a new property cjk-dir.

Let's think about vertical CJK text together with LTR text. It is reasonably rendered in a consecutiveness way, because vertical CJK characters are "by default" mapped as Top-Left/Bottom-Right in vertical mode:

But when it comes to displaying vertical CJK together with RTL text, such as Persian, it is not as readably rendered as with LTR Latin text, because CJK and Persian text are rendered bi-directionally:

However, in vertical mode, it shouldn't have had to be in that way, bacause CJK characters can be mapped Top-Right/Bottom-Left as well as Top-Left/Bottom-Right. So here comes the new property cjk-dir, it changes the CJK characters' default directionality(remap them as Top-Right/Bottom-Left), without any necessary rendering change for CJK characters themselves:

The image above shows the vertical text with cjk-dir: ltr on the left, and shows the other one with cjk-dir: rtl on the right(being obviously more readable than the left bidi one).

cjk-dir can also as perfectly slove Issue 2754, which was about RTL CJK in horizontal mode.

Start by reading Issue 2754 and the discussion here, then compare the proposed cjk-dir property with the described vertical CJK and RTL rendering cases. The issue does not name implementation files or tests; done would require a settled CSS Working Group direction and corresponding specification work.