Nearly all of them, because any language designed in the last 30 years permits user defined libraries in trivial fashion, even amazingly letting you write those libraries in the same language you’ll use it. And even more incredibly, the libraries themselves can build on other libraries! (There’s even often a standard allowing libraries to be shared by different implementations … or rather, a standard that actually standardizes things beyond the language aesthetic)

Databases follow the COBOL model of extensibility — tis for me and not for thee. It is only to be done by the vendor, and maybe some third party specialists

Every database I've ever used is extensible