New rule: sorted_collection_members

[ZevEisenberg]

I've got some more work I want to do in order to polish this up, but I wanted to get some early feedback on whether a rule like this would be accepted.

A couple of questions:

• what would be the right syntax for a one-off enable or disable on a collection literal?
• I didn't make it a correctable rule, because I didn't want to have to deal with preserving comments that may be interspersed. But it could probably be added, as long as the rule is either "can't autocorrect if there are interspersed comments" or "any comment lines stay stuck to the collection member that they are above."

Motivating use case: we have some arrays in our code, particularly in Package.swift, that look like this:

let package = Package(
  name: "Packages",
  platforms: [
    ...
  ],
  products: [
    ...
  ],
  dependencies: [
    ...
  ],
  targets: [
    // Please keep this array alphabetized!

    .target(
      name: "ACoolFeature",
      dependencies: [
        ...
      ]
    ),
    .target(
      name: "AnotherCoolFeature",
      dependencies: [
        ...
      ]
    ),
    ...
  ]
)

Instead of imploring future developers to keep a collection literal's members alphabetized, I'd like to propose an opt-in rule that can keep us honest:

// enable it once for a file or declaration, but you'd probably never want it enabled app-wide.

// swiftlint:enable sorted_collection_members
let package = Package(
  name: "Packages",
  platforms: [ // swiftlint:disable:this sorted_collection_members
    // enable or disable it ad-hoc if you need to
    ...
  ],
  products: [
    ...
  ],
  dependencies: [
    ...
  ],
  targets: [
    .target(
      name: "ACoolFeature",
      dependencies: [
        ...
      ]
    ),
    .target(
      name: "AnotherCoolFeature",
      dependencies: [
        ...
      ]
    ),
    ...
  ]
)
// swiftlint:disable sorted_collection_members

[ZevEisenberg]

Here's one possible weakness with this approach: it appears that, if a rule is disabled or not-opted-in globally, it cannot be enabled with swiftlint:enable:this sorted_collection_members on a per-literal basis. Does this sound right, and if so, is it something you'd be willing to entertain a PR to change?

[ZevEisenberg]

If that's too drastic a change for this repo, I was able to get something similar working in SwiftFormat, so I may just go with that. I'll leave this PR open until I hear from a maintainer, though.