[web] introduce NavigationSidebarTabIndicator

Summary:
The nav sidebar in the figma has a purple inset to indicate to the user which where they are in the navigation sidebar. This diff introduces that inset/indicator component

For context here is what it looks like in the figma:

Linear task: https://linear.app/comm/issue/ENG-5950/move-the-purple-inset-to-the-selected-community

Test Plan:
please see the screenshots below:

home:

community:

settings:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10979