[web] introduced ModalOverlay component

Summary:
introduced ModalOverlay component. The ModalOverlay component extracts the overlay logic from the Modal component into a separate component. Eventually we are going to want to migrate the ModalOverlay into lib

Linear Task: ENG-2038

Test Plan:
Please watch the video to see that there are no visual or performance regressions with the new ModalOverlay component

Reviewers: atul, abosh, rohan, przemek

Reviewed By: atul, przemek

Subscribers: przemek, ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5363