This is a library to be loaded by ModuleClassLoader with a purpose of extending or amending behavior of user or standard Android framework code. Instead of making changes in layoutlib one could add code here because because one wants to:
To build the library and update for the Android Studio run:
tools/vendor/google/layoutlib-prebuilt/update_layoutlib_extensions.sh
After that, the updated library can be found in prebuilts/studio/layoutlib/data/layoutlib-extensions.jar