Add ArbitraryDequeue to scalacheck module and wire ArbitraryTreeList

[krrishverma1805-web]

What this PR does

• Adds a new ArbitraryDequeue instance to the cats-collections-scalacheck module, making Arbitrary[Dequeue[A]] available to downstream users.
• Wires the existing ArbitraryTreeList into AllArbitrary and the package object, which was previously defined but not included.

Why

The TODO.md notes: "move arbitrary instances somewhere that one can import into other projects".

Currently, Arbitrary[Dequeue[A]] is only defined inline in DequeueSuite and is not available to downstream library users who may want to write property-based tests involving Dequeue. This PR makes it available through the standard import paths:

import cats.collections.arbitrary.dequeue._
// or
import cats.collections.arbitrary.all._

[krrishverma1805-web]

Hi @gemelen 👋

Would you have a chance to take a look at this when you get a moment?

This PR addresses one of the items in TODO.md — moving arbitrary
instances to the scalacheck module so they can be reused by
downstream projects. It also wires up ArbitraryTreeList which
was defined but not included in AllArbitrary.

Happy to make any adjustments based on your feedback. Thanks!

[armanbilge]

Hi @krrishverma1805-web, before we look at your PR, would you mind making a contribution to our onboarding repository? Directions are in the README.

https://github.com/typelevel/gsoc-onboarding

[krrishverma1805-web]

Hi @armanbilge , thanks for the pointer! I'll complete the onboarding and come back. 🙂

[krrishverma1805-web]

I've completed the onboarding contribution:
typelevel/gsoc-onboarding#11