Add wasmi to the list of WebAssembly runtimes

[Robbepop]

wasmi WebAssembly interpreter is missing from the list of WebAssembly runtimes.

While it is not a super efficient JIT engine like the others, it can do pretty much the same as the other engines listed there. wasmi itself can be compiled to WebAssembly to be used as a WebAssembly engine within WebAssembly which we actually even do in our company.

Link: https://github.com/paritytech/wasmi/