Yarn support has been dropped because minecraft is going to release versions that are not obfuscated at all. we had a project that generates some ts definition(https://www.npmjs.com/package/@ccbluex/liquidbounce-script-api?activeTab=versions), I started it but that's not exactly very accurate because ts and Java have a slightly different OOP model, and writing script for script api doesn't feel like writing native ts, but I think I would just wait a little bit and pick it again when obfuscation has totally dropped(I am much looking forward to it).
For a shameless self-advertising, I did have some scripts written before I got the ts generator maintained by LB maintainers, but those scripts likely won't run on current version https://github.com/commandblock2/minecraft-LBNG-types/tree/master/src , and I have yet to migrate them to our current api version.