Update the Post Meta Modal example

[ndiego]

This PR is a follow-up to #102 and adds/updates:

• Updated wp-env.json to include the example
• Fixed useCommand (It was not imported correctly)
• Added a custom blueprint
• Updated the example readme with more information and a gif
• Updated the main repo readme with the correct example info

[juanmaguitar]

LGTM! 🛳️

A better experience for the WP Playground link would be to directly open a post in Edit mode (here you have an example)

[flexseth]

Solid example @ndiego!