Move search text processing to a service

Problem/Motivation

Follow up to #3075695: Move search_index() and related functions to a service there are a number of text processing/tokenisation functions that can be moved to a service and deprecated. This includes:

• search_index_split()
• search_simplify()
• search_expand_cjk()

Proposed resolution

Move them to a SearchTokenizer service and deprecate.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet