feat: support `backup` packaging function

This PR adds support for the [`backup`](https://wiki.archlinux.org/title/PKGBUILD#backup) packaging function. **rationale** Additional files can be included, which may be user-modified configuration files. As-is, a package build using `cargo-aur` will overwrite any user-made changes to files that are contained in the package. With this feature, files can be defined which may be user-modified. If a file is user-modified, the packaged file will be added as [.pacnew](https://wiki.archlinux.org/title/Pacman/Pacnew_and_Pacsave#.pacnew) and not overwrite any changes made by the user. If a package is removed, and any packaged file is user-modified, a [.pacsave file](https://wiki.archlinux.org/title/Pacman/Pacnew_and_Pacsave#.pacsave) will be retained. **implementation** I extended the existing `[package.metadata.aur]` approach as to not get in the way of Cargo's default `[package]` and took a similar approach as to how the `package()` section is built up. This does duplicate that logic somewhat, but I suspect there may be other package functions that could piggy back of this approach in the future should there be a need for them. I added relevant information in terms of usage in the README.