Code block: set LTR direction for RTL languages

[sabernhardt]

Fixes #65890

Testing Instructions

1. Set your site language to a right-to-left language (and set your profile language to match the site).
2. Go to the post editor.
3. Add a Code block and insert some code.
4. Save changes.
5. View the direction and alignment both in the editor and on the front.

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[sabernhardt]

In addition to the ltr direction, the block seems to need left text alignment. WordPress admin CSS adds unicode-bidi: embed without the text alignment, but I did not notice a need for that property with the one code example.

Note that if your active theme is a classic theme, the admin CSS could affect code display within the post editor.

[t-hamano]

Thanks for the PR! Similar styles exist for the HTML block too, and I think this PR makes sense.

[t-hamano]

LGTM 👍