Do not reorder existing FileCopyrightText lines

[scy]

Currently, reuse annotate seems to reorder existing SPDX-FileCopyrightText lines by year, followed by copyright holder.

However, I would argue that it should instead only order by year, and leave existing lines alone. When adding a new one, it should be placed at the right spot chronologically, but if there are already other lines with the same year, the new one should simply be added below them.

I have two reasons for suggesting this:

1. The file might have one or more "main" and one or more "secondary" copyright holders. For example, Bob Builder is the lead developer for the file, but Alice Avocado has worked on significant parts as well and is a copyright holder, not just a contributor. reuse would always list Alice first, which might feel wrong to Alice, Bob, or both.
2. Adding a single contributor should be a single-line change, not potentially lead to multiple lines of diff output because reuse reordered the existing tags.

To fix this, I propose to change reuse annotate's default behavior as described above. Additionally, we could introduce a --alphabetical flag that, as before, sorts contributors alphabetically within the same year.

I'm a Python developer, I could try to create a pull request for this if you're interested, but I'd like to hear your opinions first.