Improve moveByCharacter for rtl text

[ranyadshalom]

Why

Users expect the arrow keys to move the cursor at the direction the arrow is pointing to. However, in right-to-left languages, since the text is written right-to-left, the cursor currently moves in the OPPOSITE direction of the arrow.

This change flips the direction of the movement if the whole line is made out of rtl language characters. While this isn't a thorough treatment for rtl support (the behavior is as before for lines of mixed rtl/ltr characters), it is a low hanging fruit that improves the experience by a lot for the standard case of writing in one language.

WDYT?

What changed

If a line contains only rtl language characters, arrows will move at the directions they point to.

Test plan

1. I tested with the local npm dev application on the following strings:

זה טקסט בעברית לבדיקה.
هذه هي اللغة العربية للاختبار.

זה טקסט with English.
And this is מעורבב גם כן.
ועכשיו מילה yes ומילה no.

2. made sure the behavior for English stayed the same.
3. ran the test suite

Example

example.mov

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If accepted, I can also add some automated test.