refactor(rust): add default value for async usage

[vados-cosmonic]

This commit introduces a default value for the --async option as used with Rust bindgen.

At present when using wit-bindgen the --async argument is required:

wit-bindgen rust test.wit --out-dir output
error: the following required arguments were not provided:
  --async <ASYNC>

Usage: wit-bindgen rust --async <ASYNC> --out-dir <OUT_DIR> <WIT>

For more information, try '--help'.

[dicej]

Thanks!