[web] introduce app list header

Summary:
This diff introduces the app list header. This component will be used in the app list that I will introduce in subsquent diffs

For context here is what the app list header looks like in the figma designs:

Linear task: https://linear.app/comm/issue/ENG-5962/create-new-app-list-card-section

Depends on D10436

Test Plan:
Please note that the placement of the app list is still getting fleshed out. The focus of this diff is just the UI/UX of the app list header

Please see the demo video below

Reviewers: atul, rohan, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10437