Ignoring convention that `--` separate non-flag arguments in bash (apart from last `--` for fire flags)

Beyond the fact that fire uses the last `--` to separate flag argument from command arguments, it seems that previous `--` are not parsed correctly. At least according to the common convention that a double dash ( -- ) is used in most Bash built-in commands and many other commands to signify the end of command options, after which only positional ("non-option") arguments are accepted ([source](https://unix.stackexchange.com/a/11382)).

I would like to use this to pass some other commands to a fire command, without the flags targeted to the extra command being parsed by fire. Here is a minimal example `test.py`:

```python
import fire

def foo(*args: str, flag: int = 2) -> None:
print(f"{args=} {flag=}")

fire.Fire(foo)
```

Current behavior:
```bash
$ python test.py -- other args 1 2 --flag 3 --
args=('args', 1, 2) flag=3
ERROR: Could not consume arg: --
Usage: test.py -- other args 1 2 -

For detailed information on this command, run:
test.py -- other args 1 2 - --help
```

Expected behavior:
```bash
python test.py -- other args 1 2 --flag 3 --
args=('args', 1, 2, "--flag", 3) flag=2
```

Would you be fine with updating the parsing rules in https://github.com/google/python-fire/blob/6cf45c663075c96b20dd0dfa733c2374545a4ad6/fire/core.py#L376 to consider all remaining args as positional arguments if it encounter a `--` ? I could work on a PR if there is no major blocker.