[lib] Add userStoreMentionSearchIndex selector

Summary:
This diff adds new SearchIndex selector for user mentioning. We need to use SentencePrefixSearchIndex rather than SearchIndex, because SearchIndex catches keywords rather than whole text prefix, so when user would type whole username and then input a space, the tooltip would never close.

Depends on D8900.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8901