fix(process): return exit code instead of signal

[pysan3]

It looks like the first argument of on_exit is the exit code of the process.

:h uv.spawn()

uv.spawn({path}, {options}, {on_exit})                              *uv.spawn()*

                Parameters:
                - `path`: `string`
                - `options`: `table` (see below)
                - `on_exit`: `callable`
                  - `code`: `integer`
                  - `signal`: `integer`

[rcarriga]

Ah nice thank you!