cargo add should maintain sorting of features

[Muscraft]

Problem

When you run cargo add <package> -F <feature> when the dependency already has features that are sorted alphabetically, the new features get added to the back instead of in alphabetical order. This is unexpected and different from what happens when you add a dependency to a sorted dependency list, where it respects being sorted (cargo code).

Example

Reproduction:

1. Add below to [dependencies]

   snapbox = { version = "0.5.8", features = ["diff", "harness", "path", "term-svg"] }

2. Run cargo add snapbox -F cmd
3. See that cmd is not added to the front of the list

Proposed Solution

Cargo should keep features sorted alphabetically when encountered.

Notes

No response

[epage]

For implementing this, I'd ask that a new test get added in its own commit, showing the current behavior. A follow up commit within the PR would then add sorting-if-sorted of features, updating the test to show this new behavior.

We need to make sure we have test cases both for "unsorted, keep it that way" and "sorted, keep it that way".

[rohansx]

@Muscraft @epage I would like to take on this task

[rohansx]

@rustbot claim