feat: Add a Preview Feature to check the text after formatting it.

[thepiyush-303]

Brief Title

I need to provide users with a preview option for formatted messages before sending them, So that they can review and ensure the message appears as intended.

Fixes #890

Video/Screenshots

402599737-a2ee7a8b-0b1f-4ecf-b52d-6f6f41752d54.webm

PR Test Details

I'm not sure if this is necessary, but as a user, I wanted this feature in RC, so I started working on it without considering the requirement. If it turns out that it's not needed, please leave feedback on my changes.

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-<pr_number> after approval. Contributors are requested to replace <pr_number> with the actual PR number.