CharacterDirectionOfLocale: Returns "ltr" for scripts without any character direction

[anba]

ICU4C's uloc_isRightToLeft doesn't seem to provide this information, but at least ICU4X's LocaleDirectionality has three possible return values:

• Direction::LeftToRight
• Direction::RightToLeft
• None for scripts like Zzzz or Zyyy.

See also https://docs.rs/icu/latest/icu/locid_transform/struct.LocaleDirectionality.html#method.get.

I'm not sure if there's an easy way to detect the unknown direction case in ICU4C, so I don't know if there's much support to support this case in the spec. But I guess we could at least document this case more clearly in the spec by adding an <emu-note> to CharacterDirectionOfLocale to describe that "ltr" is returned even when no direction is known.