Add meta regions

Add the meta regions, Header and Footer.

Both meta regions will be theme-agnostic (not dependent on any theme) and edited with the Layout Builder.

This will greatly simplify Drupal theming because header regions like "Header", "Primary Menu", "Secondary Menu" and "Search" and footer regions like "Footer Top" and "Footer Bottom" will become redundant because we will use the Layout Builder to layout just one single Header region and just one single Footer region with any block we choose in a theme-agnostic way.

• Drupal menus will become mobile only (vertical) or responsive (horizontal-vertical) by user choose per each menu, hence menus will also become theme agnostic.
• Drupal themes will effect only the middle part of a webpage (the one that comes between a header and footer) and thus would be extremely simpler to create, maintain and override from a sub theme and themers could focus on the more important aspects of theming which are basic page layout and design and especially margins, typography, readability, acceesibility, etc.

Meta regions will make Drupal theming extremely easier and many more people will contribute themes for Drupal.