You either need to rebuild the harness every time you want to make a change, or a lot of deliberate effort is required to maintain an API surface for extensions, distinct from internal implementations, or you can embed an interpreter like Lua, Python, or... JavaScript. Or you could instead go the Pi route and use an interpreted language, and just load extensions into the interpreter, alongside the program itself. When one of the main goals is extensibility, the latter seems like the obvious choice.