feat(chat): add markdown formatting and code block support

[mishraditi]

Before

In Jitsi Meet, chat messages were displayed as plain text.
There was no support for bold or italic text, inline or block code, syntax highlighting, or formatting controls in the chat input.
URLs and emojis were handled, but messages otherwise appeared unformatted.

After

This PR enables markdown-based formatting in chat with easy-to-use tools.

What’s added:

• A chat input toolbar to insert bold, italic, inline code, and fenced code blocks
• A code block popup for writing multi-line code, optionally selecting a language, and inserting it into the message
• Markdown rendering that supports:
  Bold and italic text,
  Inline and fenced code blocks,
  Syntax-highlighted code blocks (using highlight.js),
  Automatic URL-to-link conversion,
  Existing emoji handling (unchanged)

Screenshot

Chat.Screenshot.mp4

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[damencho]

We will discuss this with the Design team and will come back to you. Thank you.

[mishraditi]

Sure, I’ll wait for your response.