WP-CLI: Add block preview support to the block scaffolding template

[pento]

Since we want all blocks to have previews, it'd be useful to include block preview support in the WP-CLI block scaffolding template, so new blocks don't accidentally forget it (like I did in #13905).

Changes proposed in this Pull Request:

• Add an example property to the settings object in the block index.js mustache template.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This is an enhancement to the existing wp jetpack scaffold block WP-CLI command.

Testing instructions:

• Run wp jetpack scaffold block "My Block".
• Confirm that settings.example exists in the block's index.js.
• Confirm that a preview for the block shows in the block selector.

Proposed changelog entry for your changes:

• CLI: Add block preview support to the wp jetpack scaffold block command.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: November 5, 2019.
Scheduled code freeze: October 29, 2019

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