User mentions: add RTL support

[bluefuton]

Enhancement to user mentions functionality added in #24287.

Add support for RTL languages by inserting the username in the correct spot.

We can use the isRtl selector.

[github-actions]

This issue is stale because it has been 180 days with no activity. You can keep the issue open by adding a comment. If you do, please provide additional context and explain why you’d like it to remain open. You can also close the issue yourself — if you do, please add a brief explanation and apply one of relevant issue close labels.

[github-actions]

This issue is stale because it has been 180 days with no activity. You can keep the issue open by adding a comment. If you do, please provide additional context and explain why you’d like it to remain open. You can also close the issue yourself — if you do, please add a brief explanation and apply one of relevant issue close labels.