Components: Add in-progress state to checklist tasks

[tyxla]

Changes proposed in this Pull Request

• Introduce an in-progress state to checklist tasks.

Background

We need this for the Jetpack checklist project, see p5XAZ9-2c7-p2 for more details.

Preview

Testing instructions

• Checkout this branch and build Calypso.
• Go to http://calypso.localhost:3000/devdocs/design/checklist
• Verify the new third task looks and works well.
• Verify the existing 2 tasks still look and work well, too.
• Test the checklist in an existing site, and verify it still works well and unchanged.

[matticbot]

Test live: https://calypso.live/?branch=update/checklist-task-in-progress-state

[jeffgolenski]

This is looking tremendous @tyxla - Only a small change request. Can we change "Your setup list" to "Your Jetpack setup progress" above the progress indicator? This way folks know it's not just a random list, but a progress list for Jetpack.

EDIT: The above comment is actually not related to this PR. My Mistake. Ignore it for this merge.

[ramonjd]

checked out the demo and gave the checklist a spin just in case LGTM

[tyxla]

Thanks, folks!

[tyxla]

Can we change "Your setup list" to "Your Jetpack setup progress" above the progress indicator? This way folks know it's not just a random list, but a progress list for Jetpack.

@jeffgolenski I'm going to follow up in another PR with this.

[tyxla]

@jeffgolenski done in #31990.