Implement `@since` and `@unstable` attributes

[peterhuene]

See WebAssembly/component-model#332

We need to integrate this into the supported WIT subset of WAC.

[ctaggart]

A lot of the standard WIT definitions have @since added, so wac targets does not currently work with them.

They have been added for at least 6 months:
https://github.com/WebAssembly/wasi-cli/blame/main/wit/command.wit

example error:

~/ms/StarlingMonkey15/typescript> wac targets --world typescript-system --wit wit ../typescript-system/target/wasm32-wasip1/debug/typescript_system.wasm
error: failed to parse dependency directory: wit/deps

Caused by:
    0: failed to parse package: wit/deps/wasi-cli-0.2.2
    1: expected `world`, `interface` or `use`, found `@`
            --> wit/deps/wasi-cli-0.2.2/package.wit:3:1
             |
           3 | @since(version = 0.2.0)
             | ^