[Feature] --toml flag for lx update

[mrcjkb]

To conveniently upgrade packages, similar to cargo-edit's cargo upgrade, we could add a --toml flag to the lx update command.
This would result in packages in the lux.toml being upgraded to the latest version, before updating the packages in the lockfile.

I think a --toml flag is more ergonomic than a separate update command, because you will need to update the lockfile whenever you upgrade.
@vhyrro what do you think?