Remove experimental from terminal image support

[Tyriar]

Follow up from #182443

Ideally before we do this the following will be done:

• xterm-addon-image is moved into the xterm.js core codebase. This is important because the addon uses internal APIs which are easy to break between versions, if it's in the core codebase we can strongly type the internal API usage. Move into core codebase jerch/xterm-addon-image#49
• Documentation added to vscode-docs.
• Serialization support is implemented in the xterm-addon-image. See Support 1337 escape sequence to show inline images xtermjs/xterm.js#614 (comment) and quick demo of complex serialization jerch/xterm-addon-image#48. This isn't critical but a nice to have, we could document this limitation on the docs website.

[Tyriar]

This was done