Add Post Comment block

[david-szabo97]

Description

Related: #24101
Laying the foundation for a block-based post comment building. Adds a Post Comment block which requires a Comment ID input for now. The Post Comment block provides a context for the inner blocks.

As an example, also implemented a very simple Post Comment Content block, which uses the context provided by the Post Comment block.

How has this been tested?

1. Open site editor
2. Add Post Comment block
3. Input an existing comment's ID
4. Insert Post Comment Content block into the Post Comment block
5. You should see the content of the comment

Types of changes

New feature

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[ockham]

Unit tests are failing because of missing fixtures for the two new blocks that this PR introduces. This doc has some information on how to create them 🙂

[david-szabo97]

@ockham Gotcha! Added the icons and fixtures. Tests are happy now!

[ockham]

Looking good! This will provide us with a good baseline to work on other comment related blocks.