Add decoder for RVV instructions

[jserv]

How can you test the RVV instructions?

[howjmay]

A test set own by chipsalliance is here. https://github.com/chipsalliance/riscv-vector-tests/tree/main

I think we can test instructions' behavior with this repo. However, testing decoder is harder. I think I will Spike the verify the behavior, or generating the test cases