`zig fetch`: add `--save` flag

```
--save        Add the fetched package to build.zig.zon
--save=[name] Add the fetched package to build.zig.zon as name
```
0c0b69891aAndrew Kelley committed on 11/27/2023, 2:41:00 AMparenta0c8d54
3 files changedLine totals unavailable