As I understand it, Node's Buffer is dangerous to expose from a security perspective. This is because it allows you to allocate and access uninitialised memory... though perhaps in the context of IVM this is cleared? Anyway, at the very least it's a bunch of pointer leaks they probably don't want.
Back to your original idea, providing TextEncoder and TextDecoder would be really useful not just for Rust users, but for anyone wanting to do custom memory or WASM modules. I'm in the JS crowd, but would love to have a proper way to store an ArrayBuffer in RawMemory or segments without an incredibly slow polyfill. JavaScript strings are just awful to deal with.