wkg wit update still a todo

`wkg wit update` has a `todo!()` panic.

https://github.com/bytecodealliance/wasm-pkg-tools/blob/main/crates/wkg/src/wit.rs#L152C12-L152C13

I want to be able to refer to a wit world and opt to overwrite my current set of dependencies and update my lock file. In order to workaround this, I had to use wkg get --overwrite instead: https://github.com/bytecodealliance/wasmtime/blob/main/ci/vendor-wit.sh#L45