Content Model convert "dir" attribute to "direction" style

[haven2world]

Describe the bug
Content Model's directionFormatHandler convert "dir" attribute to "direction" style, but in MDN, "direction" is not recommended.
First, "direction" style doesn't support "auto"
And even more, the "dir" attribute is still preserved, if "dir" is "auto" and "direction" is left, the RTL support will be broken.
https://developer.mozilla.org/en-US/docs/Web/CSS/direction

To Reproduce

1. add "dir=auto" on any Div
2. trigger Content Model conversion

Expected behavior
Keep "dir" as recommended