[lib] Introduce SentencePrefixSearchIndex

Summary:
Solution for ENG-4556.

This diff introduces new SearchIndex class which operates on text prefixes rather than on keywords.

Depends on D8898.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8899