doc: update WASI example to use import.meta.url
This updates the WASI documentation example to use import.meta.url
to load the WASI file.
This ensures the example follows the more common pattern of a module-relative WASI import rather than a CWD relative one.
At the sime time I've also updated the example to use the FS promises readFile and also noted some WASI binaries need to use the wasi_unstable
import name.