[Web] Added current cursor position to StateInput

Summary:
Added current cursor position in textarea input of chatInputBar to StateInput. It will be used in typeahead as we will need to track cursor position. Putting it in context provides us with extra benefit of saving cursor position when jumping between different chats and channels.
Code from this diff doesn't call anything them yet.

Test Plan:
Runned web. Everything works fine.
New property and function are visible in chat-input-bar.reat.js via this.props.stateInput.

Reviewers: tomek, kamil, atul

Reviewed By: tomek, atul

Subscribers: ashoat, abosh, atul, kamil, tomek

Differential Revision: https://phab.comm.dev/D5595