when adding dependencies, insert them at the correct ordering index in the list

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Feature Request

As a python dev team member I'd like to avoid a merge conflict when two users add a different dependency to the project.
Currently all are appended to the end of the dependency list. This causes merge conflict when merging two branches with different dependencies added.
A simple solution to minimize this, is to keep the dependency list ordered alphabetically.

Ideally all new projects would use sorted lists by default, but runtime overhead and existing unordered dependency lists should be considered.

• Add new configuration option: dependencies.sort = false
• If the above option is true, change poetry add behaviour to sort all dependency lists by pretty_name and use binary search to insert the new dependencies.

Let me know if this is acceptable and I'll start working on a PR.

The issue names the poetry add entry point but no files or tests. Start by tracing its current dependency-list insertion behavior, then determine how the proposed dependencies.sort option should interact with existing unordered lists. Done means the ordering behavior and configuration scope are defined well enough for implementation and testing.