Build Tools: Add project root composer.json file.

This commit adds the following Composer package developer dependencies:

• phpcompatibility/phpcompatibility-wp
• wp-coding-standards/wpcs
• dealerdirect/phpcodesniffer-composer-installer

This also adds two composer scripts:

• composer lint to "lint" the PHP files using PHPCS
• composer format to "format" the PHP files using PHPCBF

Props jrf.
See #3294.