feat: support `backup` packaging function

[sjugge]

This PR adds support for the 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 and not overwrite any changes made by the user.

If a package is removed, and any packaged file is user-modified, a .pacsave file 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.