[Feature Request] Checklist Block Separate from Form Block

### Impacted plugin

None / Other

### What

Allow checkbox options on a site without being tied to a form block, similar to the Task block on P2

### How

Similar to how the Task block creates a checkbox interface on the live P2, we could have a task or checklist block for WordPress sites which will allow the site visitors to check off tasks locally, but without impacting the broader site.
Screenshot of Task block from P2 for reference:

The issue names no files, tests, or entry points. Start by locating the existing Task block and Form block implementations in Jetpack, then define the standalone block's visitor-local checkbox behavior and verify that checks do not affect site-wide or form state.

Written by the indexing model from the issue text.