#4128954 FSE: Add plans grid modal.

Changes proposed in this Pull Request

This is a rough & dirty PR just makes plans grid display on block editor. All issues (styling/domains/data/etc) to be fixed in other PRs.

Testing instructions

After checking out this branch, run yarn once to install all the dependencies & recompile the packages/plans-grid package.

• Set define( 'A8C_FSE_PLANS_GRID_ENABLE', true ); in .config/sandbox.php on your sandbox.
• Run yarn dev --sync on appsfull-site-editing.
• Go to /wp-admin/post-new.php?post_type=page on your sandbox.

Known issues

• Plans grid could not display price. To fix in another PR.

Screenshots