Search: Enable synonyms

#### Is your feature request related to a problem? Please describe.
Specific use case:

If a site has posts about a person who then transitions gender and thus changes their name, new posts containing only their new name will not come up if someone searches for their dead name.

#### Describe the solution you'd like
Enable synonyms so that searches for both names give the same results.
https://www.elastic.co/blog/boosting-the-power-of-elasticsearch-with-synonyms

#### Additional context
3537009-zen

6202-gh-Automattic/jpop-issues

No repository files, tests, or entry points are named. Start with the linked Elasticsearch synonyms article and the repository's search implementation, then determine how synonyms should be configured and maintained; done means searches for both names return the same relevant results.

Written by the indexing model from the issue text.