Add bidi support

Regardless of the surrounding page direction, the content written in trix should adjust the direction based on the user input. This can be achieved by adding bidirectional text support. This way, without explicitly setting the direction, anyone can write RTL or LTR or even mixed content (common when there is an article in RTL scripts that include code or Latin quotations).

I might be able to add it to the project if it is approved.

Any consideration?

No file, test, or entry point is identified. Start by locating Trix's existing content-direction handling and define expected behavior for RTL, LTR, and mixed input independently of the surrounding page direction. Done should include documented behavior and coverage for the supported direction cases.

Written by the indexing model from the issue text.