This commit migrates `scroll_util.js` to typescript. I made a helper
type `JQueryOrZJQuery` for the argument of `get_scroll_element` instead
of adding a `__zjquery` property to the global JQuery object because it
isn't being used anywhere outside of this function and hence it makes
sense to have a little helper type than to add the property to the global
object.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.
c7b8f0658b