WikibaseLexemeCirrusSearch will be a parallel extension to WikibaseCirrusSearch to implement lexeme-related functionality.
Patches for WBCS that we'd need to repeat for Lexeme:
[x] https://gerrit.wikimedia.org/r/c/integration/config/+/432923
[x] https://gerrit.wikimedia.org/r/c/integration/config/+/486170
[] https://gerrit.wikimedia.org/r/c/mediawiki/tools/release/+/489362
[] https://gerrit.wikimedia.org/r/c/mediawiki/extensions/+/432751
[] https://gerrit.wikimedia.org/r/c/mediawiki/vagrant/+/489967
[] https://gerrit.wikimedia.org/r/c/translatewiki/+/492249
[] https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/490643
[] https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/490644
[] https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/490645