Add custom blocks and update build process

This release does two primary things:

1. Updates the Webpack configuration to make it work more smoothly with the WP block editor by adding a separate entry point for blocks (allowing us to use the WordPress Babel preset instead of the default one).
2. Introduces three custom layout blocks -- notifications, callouts, and sidebar -- and updates front-end and editor styles to align with these blocks.

This release also deprecates the styles for the non-block-syntax notification and callout (module) components.

Notable Changes

Fixed

• Fix z-index and text color for new cover blocks allowed content.
• Child blocks should not inherit center alignment from parent blocks.

Added

• A "sidebar" block in the block editor to display content in a weighted two-column layout with options for the "sidebar" to sit to the left or the right.
• A custom "callout" block in the block editor to display a heading & content-style element.
• A custom "notification" block in the block editor to display a text & button-style notification element.

Changed

• Update notification and callout styles for new block syntax.
• Move block registration methods to the theme setup class.
• Add separate webpack environment config for WP blocks.

Deprecated

• Old-style module (callout) styles.
• Old-style banner notification styles.