preview_message_area: Use simplebar scrollbar.
preview_message_area: Use simplebar scrollbar.

Add `data-simplebar` attribrute to `preview_message_area` div in
`templates/zerver/app/compose.html`.

This will cause preview_message_area div to use simplebar scrollbar
instead of normal scrollbar.

Fixes #16468.

rht/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,rht/zulip,hackerkid/zulip,kou/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,punchagan/zulip,zulip/zulip,kou/zulip,showell/zulip,punchagan/zulip,kou/zulip,andersk/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip