* Add stemming support for fields
* Add tests for stemming
* Check if the stemmer is loaded correctly
* Add mutex for stemmer initialization and usage
* Fix get_stemmer_mutex() function signature
* Add StemmerManager class and refactor field.cpp and index.cpp
* Add mutex for thread safety in StemmerManager
* update delete_all_stemmers()
* Update field property name from "stemming" to "stem"
